Chordate Zoology - Textbook - Y. V. Tsaryk - 2013

Chapter 1. SUBPHYLUM ACRANIA

1.1. STRUCTURAL FEATURES OF THE CLASS CEPHALOCHORDATA

We will examine the details of the external appearance of a typical cephalochordate using the lancelet Branchiostoma lanceolatum as an example. Because its body is translucent, some of the Main Features of its internal Structure are visible (Figs. 1.1, 1.2). The body is laterally compressed and bordered by a fin fold. The fold begins at the anterior end, runs along the dorsal side, and smoothly transitions into a lanceolate caudal fin, then continues as a ventral fin; right and left metapleural folds extend from the oral hood along the sides of the body, merging with the ventral fin.

The lancelet's Skin is formed by an epithelium (epidermis) and an underlying thin, gelatinous layer of Connective Tissue, the corium. Secretions from epidermal glands form a thin film (a mucopolysaccharide cuticle) that protects the lancelet's body from damage.

The notochord runs from the anterior to the posterior end of the body, and its structure in lancelets is unique. In other Chordates, the notochord exists only during Embryogenesis or has a somewhat different structure (as, for example, in sturgeons). The notochord is a complex system of transverse Muscle plates surrounded by a connective tissue sheath. The plates are isolated from each other and are connected by transverse projections only in certain places. The notochord is a muscular organ capable of regulating body turgor, meaning it acts as a hydrostatic Axial Skeleton.

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Fig. 1.1. A - STRUCTURE OF THE lancelet in a whole mount specimen;

B - general view of Branchiostoma lanceolatum

Fig. 1.2. Cytology/practical/72.html">Cross section of the lancelet through the pharyngeal region

Activation of the notochord occurs due to action potentials generated in the giant nerve fibers of the Rohon-Beard Cells of the nerve cord.

Adjacent to the notochord are 50-80 muscle segments, or myomeres, which are separated from each other by connective tissue partitions called myosepta. Myomeres are composed of striated muscle. Each is shaped like a half-cone, the apex of which fits into a recess of the preceding myomere. This design ensures the connection of the myomeres to each other and to the notochord. Contraction of the myomeres causes the body to bend horizontally. The presence of elastic tail lobes enables the forward locomotion of the individual.

The Digestive System of the lancelet is structured as follows: at the anterior end of the body, there is an oral hood with a ring of tentacles. Inside it is the Mouth, surrounded by the velum—a muscular membrane. On the anterior surface of the velum, there are thin, ribbon-like projections of the wheel organ, and on the posterior surface, short tentacles project into the pharyngeal cavity, preventing excessively large food particles, sand, etc., from entering the Pharynx (Fig. 1.3). The walls of the large pharynx are pierced by more than a hundred pairs of gill slits. The pharyngeal, or gill, slits are separated from each other by thin bars covered with ciliated epithelium.

Fig. 1.3. HEAD region of the lancelet

The gill slits lead into the atrial cavity, which surrounds the pharynx and opens to the outside via the atriopore. Water enters the pharynx through the mouth, passes through the gill slits into the atrial cavity, and exits through the atriopore. The flow of water is driven by the wheel organ and the beating of cilia covering the gill bars. At the bottom of the pharynx lies the endostyle, a groove lined with glandular and ciliated epithelium (Fig. 1.3).

From the anterior end of the endostyle, two ciliated tracts extend, encircling the mouth opening and merging at the beginning of the epipharyngeal groove located on the DORSAL SIDE OF the pharynx. The cells of the endostyle secrete mucus. The beating of the cilia of the endostyle and gill bars drives the mucus up the pharyngeal walls toward the epipharyngeal groove. Along the way, this mucous "net" traps food particles carried by the water current. The ciliated epithelium of the epipharyngeal groove directs the mucus backward, where it enters the foregut.

The posterior part of the pharynx narrows sharply and transitions into the intestine, which ends at the anus. Immediately behind the pharynx is the hepatic caecum, the walls of which secrete digestive Enzymes. Digestion occurs both within the hepatic caecum and throughout the entire intestine.

The Circulatory system OF cephalochordates is closed. Blood flows only through vessels that have their own walls. In its general plan, it is close to the circulatory system of aquatic vertebrates, but it lacks a Heart (Fig. 1.4).

Fig. 1.4. Diagram of the circulatory System of the lancelet

Beneath the pharynx runs a large vessel, the ventral aorta (aorta ventralis), through which venous blood flows toward the anterior part of the body. More than a hundred branchial Arteries branch off from it. Blood flow is driven by the pulsation of the ventral aorta and the bases of the branchial arteries. The branchial arteries empty into the paired roots of the dorsal aorta, from which two branches, the carotid arteries, extend forward. These supply blood to the anterior part of the body. In the posterior region of the pharynx, the aortic roots merge into the dorsal aorta (aortadorsalis), which runs beneath the notochord to the tip of the tail. Arteries branch off from it to supply blood to all PARTS OF THE body. Venous blood passes through a capillary system and is collected from the intestinal walls into the single subintestinal vein, which breaks up into capillaries again in the hepatic caecum, forming the hepatic portal system. The capillaries merge into a short hepatic vein (vena hepatica), which empties into the sinus venosus. From the anterior and posterior ends of the body, blood is collected into paired anterior and posterior cardinal Veins

(vena cardinales anterior et posterior). On each side of the body, they merge into the right and left ducts of Cuvier, which empty into the sinus venosus (an expansion). The ventral aorta originates from the sinus venosus.

Thus, cephalochordates have only a single circulatory loop. The blood contains no respiratory pigments. Although the well-developed pharyngeal apparatus filters food particles efficiently, its role in gas exchange is not critical. The thin integument does not impede gas diffusion, so the blood is oxygenated not only in the branchial arteries but also in all other Vessels of the body. Consequently, the oxygen concentration in the arteries and veins is nearly identical.

The excretory system of cephalochordates is similar to the nephridial system of Annelids. Up to a hundred pairs of nephridia are located above the pharynx. A nephridium is a short, curved tube that opens into the atrial cavity above the upper end of the gill slit (Fig. 1.5). Almost the entire nephridial tube is embedded in the body cavity—the coelom (remnants of the coelom in the lancelet persist as two cavities on the sides of the upper pharynx, at the Base of the endostyle and the metapleural folds). Within the coelom, the tube has several openings called nephrostomes, each surrounded by specialized cells known as solenocytes.

Fig. 1.5. Nephridium of a lancelet

A solenocyte is a club-shaped Cell; its long stalk contains a narrow canal enclosing a beating flagellum. Capillary glomeruli are located in the walls of the nephridial body, filtering organic waste products. Through these glomeruli, waste products enter the coelomic cavity, and from there, pass into the body of the solenocyte and the canal of its stalk. The beating flagellum drives the waste products from the solenocyte canal into the nephridial tube, and then further into the atrial cavity. Due to the constant flow of water in the atrial cavity, metabolites are excreted from the animal's body into the aquatic environment. This type of excretory system is efficient only for organisms with a low metabolic rate, such as cephalochordates.

Lancelets and other cephalochordates are dioecious. Up to 25 pairs of Gonads develop in each individual. They lack ducts. Mature Gametes are released into the atrial cavity through the rupture of the gonad wall, are swept away by the water current, and are discharged outside through the atriopore. The release of gametes occurs immediately after sunset. Fertilization is external.

The Ontogeny of the lancelet is typical of chordates. Its initial stages proceed very rapidly. The fertilized egg (0.1 mm in diameter) cleaves equally, resulting in a spherical blastula. About 5–8 hours after fertilization, invagination begins, forming a double-layered gastrula. Within 12–14 hours of fertilization, the mesoderm, neural tube, notochord, and coelomic pouches are formed. The coelomic pouches do not develop simultaneously, but sequentially from front to back. As they expand, each divides into a thick-walled somite, located on the sides of the notochord and neural tube, and a thin-walled lateral plate. The walls of the somites subsequently form the Sheath of the notochord and neural tube, the myomeres, and the dermal layer of the skin (cutis). The walls of the lateral plate give rise to the peritoneal layers and gut Muscles, while the merged cavities of the lateral plates transform into the secondary body cavity—the coelom. At this stage, the embryo hatches from the egg membrane and enters the aquatic environment as a larva about 3 mm long. The larva swims using the cilia that cover its body.

About 30–36 hours after fertilization, a depression forms on the left side of the anterior part of the body, into which the gut cavity eventually breaks through (the mouth develops asymmetrically). On the ventral side of the body, behind the mouth, a single gill slit appears, followed sequentially by 14 more, which later shift to the right side. On this same right side, an additional eight gill slits break through. After some time, the 14 previously formed gill slits shift back to the ventral side, their number reduces to eight, and they move to the left side of the body. The mouth shifts to the ventral side of the body, and the larva becomes symmetrical. The anus breaks through, and the endostyle forms at the bottom of the pharynx. At this point, the larva begins to feed. The number of gill slits increases. Outgrowths called metapleural folds arise on the sides of the body above the gill slits; they grow and fuse beneath the animal's belly, forming the atrial cavity. Nephridia are formed. The larval stage lasts for three months. Then, the larva settles to the bottom, and an oral hood with a ring of tentacles and gonad primordia appear around its mouth. At one year of age, the lancelet reaches 3 cm in length, at two years—4 cm, and at four years—up to 8 cm. Sexual maturity is reached in the 2nd or 3rd year of life.

The Organization OF THE larval stage of the lancelet is believed to resemble that of the hypothetical ancestor of vertebrates. The Central Nervous System is represented by a neural tube located above the notochord, with its anterior end stopping slightly short of the end of the notochord (hence the name of the class Cephalochordata); the anterior end of the notochord extends beyond the anterior end of the neural tube. Externally, the neural tube is not differentiated into a Brain AND SPINAL cord, but internally and functionally, such a division exists. The anterior part of the neural tube influences all reflex activity of the animal. Destruction of the anterior part of the tube leads to the impairment of

coordination of movement: the narrow cavity of the tube—the neurocoel—forms a small expansion here, which is considered the primordium (or remnant) of a brain ventricle. In larvae, this cavity is connected via the neuropore (an opening) to Kölliker's pit, located on the body surface (an Olfactory Organ); the neuropore later closes. At the bottom of the neurocoel expansion, there is an infundibulum formed by ciliated and secretory cells. It is likely a motion receptor or the primordium of the hypophyseal system. At the anterior end of the tube, There is a pigment spot—an unpaired ocellus, considered a remnant of the Organ of Balance (Fig. 1.3, 3). Two pairs of Cranial Nerves arise from the anterior end of the neural tube, providing innervation to the anterior part of the body.

Two pairs of nerves (right and left dorsal and ventral) arise from the neural tube in each body segment. Additionally, each segment contains Nerve Cells that form reflex arcs, and Neurons that connect the right and left sides of the animal's body. Characteristic features include Rohon-Beard cells, whose dendrites are connected to sensory nerves from the skin, and whose axons connect to the Rohon-Beard cell located above; they ensure the transmission of impulses to the Forebrain.

In the 6th–11th and 39th–61st segments of the lancelet's neural tube, there are giant Ovsiannikov-Rhode nerve cells, which establish an intersegmental chain connection from head to tail (along the top of the neural tube) and in the reverse direction (along the bottom of the neural tube).

The presence of vestigial structures in the anterior region of the central nervous system indicates that it was more complex in the ancestors of cephalochordates.

The Sensory Organs of cephalochordates are simple. Mechanical (tactile) stimuli are perceived by nerve endings in the Superficial layer of the skin. Encapsulated nerve cells that perceive chemical stimuli are also located there, lining Kölliker's pit. Within the neural tube, in the region of the neurocoel, are Hesse's eyespots, each consisting of a sensory cell adjacent to a curved pigment cell. Hesse's eyespots detect sunlight penetrating through the animal's skin. They function as photorelays, registering the depth to which the body is buried in the sediment.

Many nerve cells are capable of secreting substances (neurosecretions) involved in the transmission of nerve impulses.

The presence of the myochordal complex, along with the CHARACTERISTICS OF THE nervous system and sensory organs, accounts for the relatively high mobility of cephalochordates. Since the animal feeds passively, this mobility does not serve for food searching, but rather facilitates the seasonal Migrations of the lancelet over distances of several kilometers.