Invertebrate Zoology: Study Guide - T. A. Dauda 2014

Multicellular Animals
Molluscs
Class Cephalopoda

The Class Cephalopoda comprises about 730 species of highly evolved predatory Mollusks—aquatic organisms that inhabit primarily warm seas. They are typically large animals ranging in length from 1 cm to 4 m, while the deep-sea squid can reach up to 18 m, 10 m of which consist of its tentacles. In most forms, the shell is reduced to small plates concealed beneath the mantle and serving as internal skeletal structures.

The body of cephalopod mollusks consists of a HEAD and a trunk (see Fig. 44). The FOOT is modified: its anterior part has shifted to the head and transformed into tentacles, which number either 8 (in octopuses) or 10 (in squids and cuttlefish). The tentacles, or arms, surround the Mouth opening. They are lined with rows of strong suckers and possess powerful musculature. The posterior part of the foot has transformed into a funnel located on the ventral side of the body behind the head. The funnel is shaped like a conical tube that opens widely into the mantle cavity at one end and into the Water at the other.

Fig. 44 Cephalopods:

1 — octopus; 2 — squid; 3 — cuttlefish with dissected mantle cavity.

The head bears the tentacles; the oral fissure, protruding from which are powerful horny jaws resembling a parrot's beak; and a pair of eyes equipped with a cornea, iris, lens, vitreous body, and retina.

The trunk is enveloped by the mantle like a bag. On the dorsal side, the mantle fuses with the trunk, while on the ventral side, it encloses a spacious mantle cavity that communicates with the external environment through a wide transverse slit situated behind the funnel between the trunk and the free edge of the mantle. The mantle wall features powerful musculature.

The muscular mantle and funnel function as a peculiar jet propulsion system. When the mantle walls contract, the transverse slit closes, and water is forcefully expelled through the funnel. Consequently, the mollusk's body is propelled backward by a sudden jolt. This is followed by Muscle relaxation, the opening of the slit, and the influx of water into the mantle cavity. The mantle contracts again, giving the body another thrust. The rhythmic contractions and relaxations of the mantle Muscles enable cephalopods to swim at high speeds while simultaneously ensuring a supply of fresh water to the gills.

The integuments of cephalopods contain pigment Cells which, upon receiving appropriate signals from The Nervous system, alter their shape, allowing the mollusk to change its coloration to match the surrounding environment. In addition, many cephalopods possess an ink sac, the contents of which are discharged into the water via the hindgut. When defending itself, the mollusk expels a dark fluid and escapes behind the resulting screen.

Among all invertebrates, cephalopods possess the most complex nervous system. All ganglia characteristic of mollusks merge to form a Brain, from which nerves extend to various Organs. The advanced Development of the nervous system enables these mollusks to engage in active locomotion, prey pursuit, and other adaptive behaviors in a constantly changing habitat. Cephalopods feed on aquatic animals, capturing them with sucker-bearing tentacles and tearing them apart with their beak-like jaws.

Cephalopod mollusks are dioecious. They reproduce by laying large eggs on underwater objects. Development is direct: a miniature adult-like mollusk hatches from the egg.

Among extant cephalopods, the best-known representatives belong to two orders: Decapodiformes (decabrachians) and Octopodiformes (octobrachians). The order Decapodiformes comprises pelagic animals characterized by the presence of 10 tentacles, two of which are specialized prehensile arms that are longer and equipped with highly developed suckers. Representatives of this order, such as squids, swim rapidly through the water Column using jet propulsion. They have an elongated, torpedo-shaped body with triangular fins at the posterior end. They feed primarily on fish, pursuing shoals of herring by the hundreds. Squids are harvested in large quantities for their delicious and nutritious meat.

Other decapodians, such as cuttlefish, also move by jet propulsion. These are relatively small mollusks with a broad, flattened body and fin margins forming a border around it. Their ink sacs are well developed, and their contents are used to manufacture natural sepia ink.

Mollusks belonging to the order Octopodiformes have a sac-like body and 8 similar tentacles. They are predominantly benthic dwellers incapable of sustained swimming, moving along the bottom by means of their tentacles. Leading a nocturnal lifestyle, they hide by day among stones, in rock crevices, and so forth. Representatives include argonauts and octopuses. The largest octopuses can reach lengths of up to 1.5 m. Their meat is edible, making octopuses an object of commercial fisheries.

Phylogeny of THE PHYLUM MOLLUSCA

The embryonic development of mollusks exhibits a series of features indicating their affinity with annelid worms: spiral determinate Cleavage, the method of mesoderm formation, and the trochophore larva. In The Structure of adult mollusks, the relationships between the Gonads, coelom, and coelomoducts are similar to those in Annelids.

Furthermore, the most primitive mollusks, Amphineura and Monoplacophora, display clearly defined metamerism, remnants of which persist in lower cephalopods (Nautilus) in the form of two pairs of ctenidia, Kidneys, and atria. The small number of metameric organs in Monoplacophora points to the descent of mollusks from very primitive oligomeric annelids. The recent discovery of extant Representatives of the class Monoplacophora provides a much clearer picture of the phylogeny within the phylum Mollusca itself than was previously possible.

Based on the presence of a typical unitary shell, representatives of the class Monoplacophora should be assigned to the subphylum Conchifera, among which they stand out for their high degree of primitivity. Their metameric Organization is manifested in the presence of 5 pairs of metameric ctenidia, 2 pairs of atria, 6–7 pairs of kidneys, and 8 pairs of dorsoventral muscles.

By virtue of possessing two Heart ventricles, Monoplacophora are comparable to the embryos of Bivalvia, Gastropoda, and Cephalopoda, in which The Heart initially appears as a pair of primordia lying on either side of the gut. During further development, these primordia fuse to form a single ventricle.

In some lower representatives of the class Lamellibranchia (such as *Anadara*), this fusion of primordia does not occur, and the adult animal retains two hearts.

At the very ROOT of the Conchifera branch lies the class Monoplacophora, which combines several essential traits of Lamellibranchia, Gastropoda, and Cephalopoda. A crystalline style, apart from *Neopilina*, is present in many bivalves and lower gastropods. The general similarity in heart development between *Neopilina* and higher Conchifera has been noted previously. Phylogenetic ties with Cephalopoda (Tetrabranchia) are indicated by the vestigial metamerism in *Nautilus*. A nervous system organized as longitudinal cords is characteristic of lower Gastropoda and *Nautilus*.

The primitive Monoplacophora likely gave rise to a hypothetical group of bilaterally symmetrical mollusks that combined, in the view of many zoologists, the most primitive Structural Features of modern Gastropoda and Lamellibranchia. From this group, the classes Gastropoda and Lamellibranchia diverged fan-wise, gradually growing apart in their higher and most specialized representatives. The divergence of gastropods is rooted in The Development of Asymmetry, whereas the evolution of bivalves is characterized primarily by head reduction and The formation of a bivalve shell.

Turning to lower mollusks—the subphylum Amphineura (specifically the class Loricata) and the class Monoplacophora—one can observe A large number of homologous organs in them. Subsequent evolution of mollusks was accompanied by the oligomerization of organs, which can be most clearly traced in the ctenidia.

Among cephalopods, the primitive nautilus (*Nautilus*) still retains two pairs of ctenidia, whereas in all other mollusks there is only a single pair, unless these are reduced to a single left ctenidium or lost entirely. The reduction of the right mantle organs, affecting more than just the ctenidium and characteristic of the Evolution of the Class Gastropoda, is likewise a typical manifestation of oligomerization.

QUESTIONS FOR SELF-assessment and review:

1. What general organizational features characterize the phylum Mollusca?

2. What are the natural habitats of bivalve mollusks?

3. What is the internal and External structure of bivalve mollusks?

4. Which bivalve species inhabit freshwater ecosystems?

5. What is THE ECOLOGICAL AND economic importance of bivalves in nature and human life?

6. What habitats do gastropod mollusks occupy?

7. What is the Anatomical Structure of gastropod mollusks?

8. What role do gastropods play in nature and human life?

9. Which gastropod species live in freshwater bodies?

10. Where do cephalopod mollusks inhabit?

11. What is the MORPHOLOGICAL STRUCTURE OF cephalopod mollusks?