INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

DIVISION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBDIVISION SPIRALIA (SPIRALIA)

PHYLUM MOLLUSCA (MOLLUSCA)

CLASS CEPHALOPODA (CEPHALOPODA)

These are exclusively marine Mollusks, most abundant in tropical and subtropical waters. They occur at various depths, ranging from the surface down to the deepest oceanic trenches. Benthic, nektonic, and planktonic forms are all represented among them. A total of about 650 species have been described.

Cephalopods are the most highly organized group of mollusks and among the most advanced invertebrates. This is a fairly ancient mollusk group, with roughly 10,000 fossil species known since the Early Paleozoic.

The size of cephalopods ranges from 1 cm (some cuttlefish) to 18 m (the giant squid Architeuthis dux). They are exclusively predatory, extremely agile animals.

Anatomy. Bilaterally symmetrical animals with an external or internal rudimentary shell. Their body consists of a HEAD and a trunk; the FOOT is modified into tentacles and a funnel (Fig. 245). The large head is well demarcated from the trunk and bears eyes, a crown of tentacles (arms or feet) surrounding the Mouth, and a funnel.

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Fig. 245. External anatomy of cephalopods (adapted from Akimushkin and Filippova):

a - chambered nautilus (Nautilus pompilius); b - common cuttlefish (Sepia officinalis); c - European squid (Loligo vulgaris); d - giant octopus (Octopus dofleini): 1 - funnel; 2 - arms; 3 - sheaths; 4 - hood; 5 - ocular tentacle; 6 - eye;

7 - umbilicus of the shell; 8 - head; 9 - feeding tentacle; 10 - fin; 11 - trunk; 12 - hectocotylus;

13 - umbrella; 14 - suckers; 15 - pupil

There may be 8 or 10 tentacles, and only in Nautiloidea are there about 90. The inner surface of cephalopod arms (except in Nautilus) is covered with suckers arranged in longitudinal rows. In squids and cuttlefish, the suckers are armed with horny rings featuring smooth or serrated edges; in some oceanic squids, they transform into sturdy hooks resembling cat claws, which help them capture and hold prey (Fig. 246).

Fig. 246. Diagram of cephalopod sucker Structure (from Akimushkin and Filippova):

a, b - octopuses; c - squids; d - cuttlefish: 1 — Teeth

Located on the ventral side of the head is the funnel, a conical tube tapered at the front (Fig. 247). Through it, Water is expelled from the mantle cavity during Respiration and jet propulsion, and excreta, urine, ink, and eggs are discharged to the exterior. The sides of the funnel fuse with the mantle or connect to it via locking cartilages.

The trunk is flattened in cuttlefish, torpedo-shaped in squids, and sac-like in octopuses (Fig. 245).

The trunk is covered on all sides by the mantle. Dorsally, it forms the body covering of the trunk itself, while on the ventral side between the mantle and the body wall lies the mantle cavity, which communicates with the external environment via a slit-like opening. To lock this ventral slit in cuttlefish and squids, the inner surface of the mantle bears two cartilaginous projections—studs—that fit tightly into sockets on the Base of the funnel, effectively buttoning the mantle to the body (Fig. 247).

Fig. 247. Cuttlefish (Sepia officinalis) with dissected mantle cavity (from Dogiel)

1 - distal expansion of the tentacle; 2 - suckers of the tentacle; 3 - tentacle; 4 - arms; 5 - mouth;

6 - anterior opening of the funnel; 7, 8 - anterior and posterior PARTS OF THE funnel, respectively; 9 - anus; 10 - ctenidium;

11 - excretory pore; 12 - genital aperture; 13 - gill axis; 14 - opening of the nidamental gland; 15 - ink sac duct;

16 - ink sac; 17 - fin; 18 - mantle thickness; 19 - lobes of the pigmented gland;

20 - mantle; 21 - funnel retractor Muscle; 22 - locking apparatus tubercle; 23 - mantle ganglion showing through the integument;

24 - locking apparatus fossa; 25 - olfactory pit

The mantle cavity contains the mantle complex of Organs, the anus flanked by the excretory pores, one or two genital apertures, and a pair (or two pairs in Nautilus) of feathery ctenidia. In addition, in females, the ducts of two pairs of paired and one unpaired nidamental glands open into the mantle cavity near the genital aperture (see below).

The mantle consists of two epithelial layers (external and internal) enclosing alternating layers of circular and radial Muscles. Collagenous fibers are also incorporated into these muscular layers. The alternative contraction of radial and circular muscles drives the expansion and contraction of the mantle, while the elasticity of the collagenous fibers restores its shape following muscular contraction. At the posterior end or in the middle part of the mantle, most cephalopods bear a pair of fins, which can be rhombic, as in squids, or take the form of a narrow border encircling the trunk, as in cuttlefishes (Fig. 245).

In most cephalopods, the shell is rudimentary or entirely absent, being well-developed only in the chambered nautilus (Nautilus). The external calcareous shell of Nautilus is coiled in a single plane toward the dorsal side with the apex directed forward (Fig. 248). Its cavity is divided by transverse septa into multiple chambers. The animal's body resides solely in the last and largest chamber, whereas all preceding chambers are filled with gas and partially with fluid. A thin, cylindrical extension—the siphuncle, which is an outgrowth of the visceral mass—perforates all the chambers. In remaining cephalopods, only a shell remnant persists, enveloped by the mantle to become an internal structure. In cuttlefishes, the internal shell appears as a thick, porous, elongated-oval calcareous plate (Fig. 249). In vampires and squids, the shell is reduced to a dorsal horny blade—the gladius ("sword")—which is a narrow, feather-, lance-, or needle-shaped horny plate lying beneath the Skin on the back (Fig. 249). In octopuses, the gladius is reduced to two thin cartilaginous rods, and it is absent in most argonauts and pelagic octopuses. Among extant coleoids, only the deep-sea species Spirula spirula possesses a spirally coiled internal shell consisting of 25–35 chambers traversed by a siphuncle (Fig. 249, b). It acts as a hydrostatic apparatus, enabling Spirula to inhabit deep waters and ascend to surface layers.

Fig. 248. Section through the body of a female chambered nautilus (Nautilus pompilius) (after Dogiel):

1 - lower jaw; 2 - arms; 3 - radula; 4 - upper jaw; 5 - hood; 6 - siphuncle; 7 - septum;

8 - chambers; 9 - posterior margin of the mantle; 10 - Ovary; 11 - Heart; 12 - digestive gland; 13 - anus; 14 - ctenidia;

15 - ganglia; 16 - mantle cavity; 17 - mantle; 18 - funnel

Fig. 249. Structure of cephalopod shells:

a - cuttlefish shell (Sepia officinalis) from the ventral side (after Dogiel); b - ram's horn squid (Spirula spirula) with a spiral multichambered shell, sagittal section; c - squid gladius (Todarodes pacificus) (after Naef)

1 - closely spaced septa of the dorsal phragmocone; 2 - lateral margin of the proostracum;

3 - surface of the youngest septum; 4 - posterior margin of the proostracum; 5 - rostrum; 6 - vestige of the ventral wall;

7 - siphuncular funnel; 8 - fin; 9 - shell; 10 - siphuncle; 11 - funnel; 12 - mantle cavity;

13 - anus; 14 - excretory pore; 15 - stem; 16 - pen; 17 - terminal cone

Unlike other mollusks, cephalopods possess a well-developed internal Skeleton composed of Cartilage structurally similar to vertebrate cartilage. This principal cartilaginous capsule surrounds the Central Nervous system much like the vertebrate cranium; outgrowths extend from it to enclose the eyes and statocysts (Fig. 250). Supporting cartilages are also present at the base of the tentacles, inside the fins, and within the mantle locking apparatus ("buttons"). The cartilaginous structures of cephalopods are of mesodermal origin.

Fig. 250. Cephalic capsule of the octopus Octopus dofleini (after Nesis):

1 - Blood vessel opening; 2 - eye socket depression; 3 - statocyst capsule; 4 - posterior opening of the capsule

Integument. The skin of cephalopods consists of a single-layered cylindrical epithelium underlain by Connective Tissue. The cutaneous epithelium contains numerous mucous Cells.

Mucus makes the bodies of cephalopods slippery, which facilitates their locomotion in water.

The connective tissue layer contains muscle fibers and pigment cells (chromatophores and iridocytes). Cephalopods are capable of rapidly changing

their coloration, which is regulated by the Brain as well as a network of Nerve Cells surrounding the chromatophores. Cuttlefish and octopuses use them to mimic the color and pattern of the substrate both for defense and for ambushing prey: by hiding among bottom vegetation, the animal becomes virtually invisible. Furthermore, a sudden change in coloration allows the mollusk to deter predators.

The skin may also contain specialized light-emitting organs known as photophores. In the simplest case, these are clusters of cells called photocytes, but much more complex structures also exist, resembling a hemispherical automobile headlight. Except for the luminous surface, the photophore is enclosed in a light-impermeable pigment layer, while its base is lined with a shiny layer acting as a mirror reflector. A light source—a mass of photocytes—lies within. The "headlight" is covered from above by a transparent lens, over which lies a Diaphragm composed of a layer of black chromatophores. By drawing the diaphragm over the lens, the animal can adjust the luminous intensity of the photophore or extinguish it completely. Animals use photophores to deter predators, attract prey, and recognize one another.

The musculature is well developed. In the mantle, muscles form a continuous layer resembling a cutaneous-muscular sac. The transverse muscles are the most developed; upon contraction, they press the mantle against the ventral wall of the trunk, helping to expel water through the siphon. The circular and longitudinal Muscles of the arms, as well as the muscles of the suckers, are also exceptionally well developed. In addition, there are specialized muscles in the form of powerful bundles that retract the head and move the siphon. Most cephalopods are highly active: scientists have calculated that large-sized squids can reach speeds of up to 40–55 km/h.

Body cavity. Unlike other mollusks, the coelom in most cephalopods is large and houses the Internal Organs. The genital and pericardial compartments communicate via a duct known as the water pore canal. The vampire squid (order Vampyromorpha) possesses the largest coelom, lacking even constrictions between the two compartments. In the nautilus, cuttlefish, and squids, the genital coelom forms a true body cavity containing the gonad, Stomach, and part of the intestine. The pericardial coelom, which encloses The Heart, is also large (Fig. 251). In octopuses, the parenchyma is highly developed, and the coelom is correspondingly reduced; the heart is located within the parenchyma rather than the Pericardium. Only the genital portion of the coelom is well developed, represented by a cavity surrounding the gonad. The primary Functions of the cephalopod coelom include heart protection, participation in excretion and salt balance, and the release of reproductive products. A specialized function is performed by the coelom of cranchiid squids, which is filled with an ammonium chloride solution.

Digestive System. The mouth is located on the head in the center of the ring of arms. It leads into a muscular Pharynx equipped with a pair of sturdy chitinous jaws—upper and lower—forming a beak that resembles that of a parrot (Fig. 252). The pharynx contains a Tongue with a narrow radula. Mollusks use their jaws to kill prey and tear off pieces of flesh, rather than swallowing prey whole. The teeth of the radula catch the bitten-off food fragments, grind them down, and transfer them into the pharynx. The ducts of two pairs of Salivary Glands empty into it. The secretion of the anterior glands contains digestive Enzymes, while the posterior ones often secrete venom In addition to enzymes, which paralyzes or kills the prey. Extending from the pharynx is a long Esophagus—a cuticle-lined tube passing directly through the brain, which is why cephalopods must bite off small pieces of food rather than swallow whole prey. The esophagus opens into an entodermal stomach divided into two parts: the muscular stomach proper and The Stomach cecum (blind pouch). The ducts of the digestive gland (Liver), which consists of two parts, open into the cecum. The hepatic ducts are surrounded by numerous glandular appendages known as the Pancreas. A slender intestine extends from the stomach and opens via the anus into the anterior region of the mantle cavity. The ectodermal hindgut is absent in cephalopods (Fig. 253).

Fig. 252. Squid beak (from Nesis): 1 - lower jaw; 2 - upper jaw

Fig. 251. Coelom, excretory, and reproductive systems of a female Sepia officinalis (dorsal view) (from Nesis):

1 - pericardial coelom compartment; 2 - left renal sac; 3 - oviduct gland; 4 - genital aperture; 5 - external opening of the left Kidney; 6 — gap between ventral renal sacs; 7 - renopericardial opening; 8 - renopericardial canal; 9 - right renal sac; 10 - dorsal renal sac; 11 - branchial heart; 12 - pericardial gland; 13 - ovary; 14 - genital coelom compartment;

15 - communication between the pericardial and genital coelom compartments; 16 - opening leading from the genital coelom compartment to the oviduct;

17 - coelomic sac of the branchial heart; 18 — oviduct

Cephalopods are exclusively carnivorous animals, feeding on live prey; only Nautilus occasionally consumes carrion.

Associated with the intestine is a distinctive organ characteristic of many cephalopods—the ink sac, which opens into the gut near the anus (Fig. 247). It consists of an ink gland, whose cells produce granules of the pigment melanin, and a reservoir where the ink is stored. When threatened, the animal expels a portion of the reservoir's contents through the siphon. The primary function of the ink fluid is to disorient attacking predators. After ejecting the fluid—which temporarily retains a shape resembling the mollusk itself—the animal turns pale, abruptly alters its movement trajectory, and vanishes, while the predator seizes the ink copy instead of the mollusk. The ink cloud disperses, irritating the predator's eyes and exerting a temporary paralyzing effect on its olfactory organs, thereby hindering its search for prey. The pigment of the ink sac is one of the most durable natural colorants. High-quality brown paint, known as sepia, has long been manufactured from the ink sacs of cuttlefish.

Respiratory organs consist of ctenidia (gills) located in the mantle cavity. Nautilus has two pairs, whereas all other cephalopods have only one. Each ctenidium consists of a gill axis and two rows of folded gill lamellae containing Blood Vessels: an afferent and an efferent vessel. The epithelium of the gill lamellae lacks cilia, and water Circulation within the mantle cavity is maintained by rhythmic contractions of the mantle muscles; gill ventilation becomes particularly intense during jet propulsion.

The Circulatory system in cephalopods is nearly closed, as both arterial and venous vessels are well developed. The heart consists of a single ventricle and two or four (subclass Nautiloidea) atria. Two aortas—anterior and posterior—extend from the ventricle. The anterior, or main, aorta branches into Arteries supplying blood to the head and tentacles, while the posterior aorta sends vessels to the viscera. The arteries branch extensively, ultimately forming a capillary network from which Veins originate. Venous blood from the head and internal organs collects in two venae cavae which, passing near the Kidneys, bulge into them as bunch-like extensions called renal appendages, where the blood is filtered of excretory products. The venae cavae empty into two branchial hearts—contractile sacs lying at the base of the gills. The branchial hearts pump venous blood through the gill vessels, where it is oxygenated, after which it flows via the branchial veins into the atria and subsequently into the ventricle. Blood vessels, especially arteries, possess muscular walls and pulsate, assisting the three hearts in pumping blood through capillaries, which are particularly abundant in the appendages and the posterior part of the mantle. Elsewhere, sinuses occur between the arteries and veins.

Fig. 253. Digestive System of the cuttlefish Sepia officinalis (from Dogel)

1 - pharynx; 2 - cross-section through the nerve ring; 3 - cavity of the statocyst capsule; 4 - dissected head capsule; 5 - hepatic duct;

6 - anus; 7 - ink sac duct; 8 - rectum; 9 - intestine; 10 - stomach cecum (blind pouch);

11 - stomach; 12 - pancreas; 13 - main aorta; 14 - liver; 15 - esophagus;

16 - posterior dorsal gland; 17 - salivary duct; 18 - common salivary duct

Such a sophisticated circulatory system is one of the factors enabling certain cephalopods to reach gigantic sizes. The existence of large animals becomes possible only with the presence of a capillary system, because it is solely under these conditions that the Nutrition and respiration of massive organs are ensured.

The excretory organs are highly diverse and closely associated with the circulatory and respiratory systems (Fig. 251). The excretory system proper is represented by a single pair (two pairs in Nautilus) of kidney sacs, which open at one end into the pericardium and at the other into the mantle cavity. Large blood vessels extend into the kidneys, from which the latter extract excre-

tory products. In addition to the kidneys, the excretory function is also performed by pericardial glands located in separate Regions of the pericardial coelom near the branchial hearts. Waste substances to be eliminated pass from the blood of the branchial heart into the cavity of the pericardial gland, and subsequently into the renal venous appendages, where the reabsorption of salts, Amino Acids, sugars, and other substances vital to the Organism takes place. Thus, the primary excretory organs are the renal appendages; they also perform an osmoregulatory function. Urine accumulates in the kidney sacs, with ammonia being its main component.

Fig. 254. Excretory, respiratory, and central parts of the cuttlefish circulatory system (after Nessis): Sepia:

1 - main aorta; 2 - main vein; 3 - kidney sac; 4 - vena cava; 5 - venous appendages; 6 - branchial heart;

7 - pericardial gland; 8 - anal artery; 9 - ventricle of the heart; 10 - ink sac vein; 11 - posterior artery; 12 - ventral aorta; 13 - lateral ventral vein; 14 - branchial artery; 15 - branchial lamellae; 16 - branchial vein;

17 - ctenidium; 18 - renal opening into the pericardium (renopericardial opening); 19 - external excretory opening

The Nervous System of cephalopods varies from very primitive in Nautiloidea to the most complex and advanced among all invertebrates in Coleoidea (Fig. 255). In the nautilus, the central nervous system consists of three short nerve arcs (cords) densely covered with nerve cells and lacking differentiated ganglia. All of them are located in the head around the esophagus. The cerebral arc arches over the esophagus dorsally, while the pedal and the connected pleuromandibular (pleurovisceral) arc lie ventrally. Nerves extend from these arcs.

Fig. 255. Central nervous system of cephalopods:

a - pearly nautilus Nautilus (after Beklemishev); b - cuttlefish Sepia (after Nessis, modified):

1 - preoral (prepedal) nerve ring; 2 - NERVES OF THE lamellar organ; 3 - tentacle nerves; 4 - statocyst; 5 - nerve of the anterior ocular tentacles; 6 - eye; 7 - nerve of the posterior ocular tentacles; 8 - pleurovisceral cord; 9 - ventral ganglion; 10 - cerebral cord;

11 - buccal ganglia; 12 - labial ganglion; 13 - visceral nerves; 14 - sympathetic nerve; 15 - posterior nerve of the main vein; 16 - ink sac nerve; 17 - mantle ganglion; 18 - mantle nerve; 19 - cervical muscle nerve; 20 - Olfactory nerve; 21 - pleural ganglion; 22 - cerebral ganglion; 23 - optic ganglion; 24 - Optic nerve

Unlike the nautilus, the central nervous system of other cephalopods has a complex structure. It is highly concentrated: all ganglia form a massive circumesophageal cluster—the brain, surrounded by a cartilaginous cranial capsule. The brain includes: a pair of large cerebral ganglia located above the esophagus, from which very thick and short optic nerves extend, immediately expanding to form huge optic ganglia. Anterior to the cerebral ganglia lies a small buccal ganglion that innervates the pharyngeal organs and salivary glands. Pedal, pleural, parietal, and visceral ganglia lie beneath the esophagus. Each pedal ganglion is clearly divided into two nerve nodes: the brachial (or tentacular) ganglion and the infundibular (or funnel) ganglion. The brain, especially its supraesophageal part, exhibits complex internal subdivision, with distinct zones identified that are responsible for specific types of motor reactions, complex forms of behavior, memory, etc. The cephalopod brain is the largest in volume among invertebrates.

In addition to the brain, peripheral nerve plexuses give rise to new, supplementary ganglia absent in other mollusks. The largest of these are: tentacle ganglia lying at the base of each tentacle; mantle (or stellate) ganglia innervating the mantle; and buccal ganglia innervating the salivary glands and pharynx. Furthermore, small ganglia are scattered throughout the musculature of the arms and at the base of the suckers. Due to this, severed arms of cephalopods retain The ability to perform fairly complex and specific reactions to external stimuli.

Cephalopods possess endocrine organs. These are the optic glands (absent in the nautilus)—small, rounded, paired bodies lying on the optic nerve. The Hormones of the optic gland stimulate The Development of Gonads and accessory sex glands, govern The formation of spermatophores, determine animal behaviors related to reproduction and parental care, and also participate in protecting the organism against foreign Proteins. Neurosecretory cells are found in various locations throughout the central and peripheral nervous systems, releasing neurohormones into the blood. They regulate the vegetative functions of the organism (heart rate, blood pressure, etc.).

Sense Organs are exceptionally well developed. There are statocysts, a pair of eyes, extraocular photoreceptors, olfactory pits, a subradular organ traditionally attributed a gustatory function, as well as distinct sensory cells on the arm suckers and skin. A pair of statocysts is located in the head, enclosed within separate cartilaginous capsules connected to the main cranial cartilage.

The structure of the eyes varies (Fig. 256)—ranging from the pinhole eyes of the nautilus to the highly complex eyes of Coleoidea, which resemble those of vertebrates. They are large and situated within depressions of the cranial cartilage. The base of the eye is formed by the Eyeball vesicle, covered superiorly by a transparent cornea with a small eccentric aperture through which the anterior eye chamber communicates with the external environment, protecting the eye from high-pressure effects at great depths. A circular skin fold beneath the cornea—the iris—features an aperture, the pupil, through which light rays pass to the lens. Accommodation in cephalopods is achieved not by altering the curvature of the lens, as in vertebrates, but by moving it closer to or farther from the retina using a special muscle. The cavity of the eye vesicle is filled with a transparent vitreous body, while the bottom and lateral walls form the retina, consisting of visual and pigment cells. The collection of nerve fibers extending from the visual cells forms the optic nerve, which leads to the optic ganglion. The large number of visual cells in the retina (about 165,000 in the squid Loligo) attests to the high acuity of their Vision.

Cephalopods exhibit complex behavioral patterns. First and foremost, these are manifested in reactions of preying on food and escaping from predators. Equally complex behavior accompanies Fertilization, egg-laying, and egg-guarding (in octopuses) by females. Cephalopods, especially benthic octopuses and cuttlefish, possess memory and are capable of relatively easy learning.

Reproductive System. All cephalopods are dioecious animals with clearly expressed Sexual Dimorphism. Males differ from females by the presence of a tentacle modified into a copulatory organ—the hectocotylus. In some cephalopods, such as Argonauta, the male is significantly smaller than the female.

Fig. 256. Section of the cephalopod eye (after Beklemishev): a - Nautilus; b - Sepia:

1 - cavity of the eye socket communicating with the external environment; 2 - retina; 3 - optic nerve;

4 - vitreous body; 5 - ciliary muscle; 6 - iris; 7 - anterior chamber of the eye; 8 - lens; 9 - cornea;

10 - epithelial body; 11 - external aperture of the eye chamber; 12 - sclera (cartilaginous coat of the eye); 13 - optic ganglion

The gonad and efferent ducts are unpaired (except in Nautilus) and located in the coelomic genital region (Fig. 257). Germ Cells enter the coelom and are expelled from there through the efferent ducts into the mantle cavity. In females of many species, seminal receptacles are present on the buccal cone. In males, spermatozoa are glued into packets (spermatophores) within the vas deferens, equipped with specialized structures for the timely release of sperm. The male grabs the spermatophores—which are carried out through the funnel—with his hectocotylus and transfers them into the mantle cavity of the female (in octopuses) or attaches them to her seminal receptacles (in most cephalopods).

Fig. 257. Reproductive system of the octopus Octopus dofleini (after Nessis): a — female; b — male:

1 — oviduct; 2 — female genital aperture; 3 — oviductal gland; 4 — ovary (inside the coelomic sac);

5 — ligament supporting the ovary; 6 — severed muscle; 7 — male genital aperture; 8 — dilation of the genital duct;

9 — junction of the Seminal Vesicle and Prostate Gland with the spermatophoric sac; 10 — seminal vesicle;

11 — vas deferens; 12 — aperture of the vas deferens; 13 — Testis (inside the coelomic sac);

14 — walls of the coelomic sac; 15 — prostate gland; 16 — spermatophoric sac

A remarkable adaptation for fertilization is found in small pelagic octopuses such as Argonauta, Tremoctopus, and Ocythoe. The extraordinarily large hectocotylus of males develops within a special membranous pouch where it is initially coiled into a spiral. Once the tentacle is fully formed, the pouch ruptures and the tentacle straightens out (Fig. 258). The cavity of the hectocotylized arm becomes filled with spermatophores; it then detaches from the male's body and swims off in search of a female of its species. Upon finding one, the hectocotylus crawls into her mantle cavity. There, the carried spermatophores burst, and the spermatozoa fertilize the eggs. The lost hectocotylus subsequently regenerates. Early researchers, upon discovering these hectocotyli inside the mantle cavity of females, mistook them for parasites and assigned them the generic name Hectocotylus.

Fig. 258. Paper nautilus (Argonauta argo) (after Dogiel): a — female; b — male with a formed hectocotylus:

1 — funnel; 2 — eye; 3 — expansion of the arm through which the shell is visible; 4 — shell; 5 — hectocotylus

Reproduction. Cephalopods attach their enveloped eggs singly or in clusters to various underwater objects; less commonly, the eggs are enclosed within gelatinous sacs that float freely in the water. Parental care is observed in some species. The female paper nautilus, or argonaut (Argonauta argo), broods her eggs in a brood chamber formed by the epithelial secretions of a single pair of arms, which resembles a true spirally coiled shell. Octopuses place their clutches in rocky shelters and guard them.

Cephalopod eggs are large and yolk-rich, resulting in discoidal Cleavage. Development is direct: a small, almost fully formed mollusc hatches from the egg membranes (Octopus, Nautilus, Sepia). However, in many species, the juveniles differ significantly from the adults in body shape and the presence of larval organs; hence, such young individuals are termed larvae. Often, the larvae lead a planktonic lifestyle and occur at shallower depths than the adults.

Cephalopods possess a highly developed capacity for the regeneration of damaged or lost body parts. Autotomy—the voluntary shedding of appendages when threatened—is observed in many species of oceanic squids and octopuses.

Locomotion. The modes of locomotion in cephalopods are highly diverse. The most advanced is jet propulsion, driven by the action of the mantle and funnel. When the mantle musculature relaxes, water enters the mantle cavity through a slit; upon contraction, the cavity is sealed by snap-lock mechanisms, and water is forcibly expelled through the funnel. The resulting reactive force propels the animal's body in the opposite direction. In its normal position, the funnel's aperture points forward, meaning the mollusc typically moves backward (rear end first). However, the funnel can be directed in various ways, even backward, allowing the animal to move in multiple directions, including head-first. Some small squids achieve speeds that enable them to leap out of the water and glide 50–60 m above the sea surface to escape predatory fish and dolphins. In Nautilus, jet propulsion is generated not by the mantle, but by a large, well-developed funnel. Jet propulsion can also be achieved using the arms. In certain deep-sea pelagic octopuses, all arms are connected by a web, forming a bell reminiscent of a jellyfish, which operates on the exact same principle.

Cephalopods also swim using fins, which are present in squids, cuttlefishes, and swimming octopuses. The remaining octopuses are strictly benthic; they crawl using their arms and even 'walk' along the bottom, resting on their tips.

Alongside actively swimming forms, planktonic forms also exist. Some of these have a gelatinous body and bear a closer resemblance to jellyfish than to cephalopods; others possess a thin, nearly transparent mantle devoid of muscles. These animals generally inhabit depths of 100 m or more and hover in the water Column. They exhibit neutral buoyancy, meaning their specific gravity approaches that of water. In planktonic squids, numerous microscopic vacuoles filled with ammonium chloride solution (NH₄Cl) are distributed throughout the Tissues of the mantle, head, and arms. This forms a spongy tissue that replaces muscle; the specific gravity of such Molluscs approximates that of water. These are known as ammoniacal squids, predominantly comprising deep-sea species. The most sophisticated adaptation of this type is found in oceanic squids of the family Cranchiidae—the 'bathyscaphe' squids. In these animals, the ammonium chloride solution fills the coelom, and its volume can reach up to two-thirds of the mantle cavity volume. Ammoniacal squids are mostly small animals, though some large ones exist, such as the giant squids Architeuthis and Mesonychoteuthis.

In Nautilus, as previously noted, the Chambers of the shell are penetrated by the siphuncle—an extension of the visceral mass containing the coelom and blood vessels. This is a specialized organ for buoyancy regulation. During ascent, the siphuncle secretes gas from the blood into the shell chambers, rendering the shell lighter; when descent is necessary, fluid enters the chambers.

Extant Cephalopoda are divided into two subclasses.