Lecture Notes on Invertebrate Zoology

Module 4. Phylum Mollusca and Phylum Echinodermata

Lecture Topic 17. Characteristics of the Phylum Echinodermata. Phylogeny of Invertebrate Animals

Specific structural features: radial Symmetry, Water-vascular (ambulacral) system, calcareous internal Skeleton, multi-layered epithelium, spines and pedicellariae, axial organ complex. Reproductive System, reproduction, and development. Bilateral larvae of the dipleurula type. Simplification of body Structure As a result of a sessile lifestyle. Systematics of the phylum. Ecological significance of echinoderms, their harvesting, and practical use. Origin of life on Earth, MAIN STAGES OF evolution. Phylogenetic relationships of major invertebrate phyla, connecting links. Significance of invertebrates in nature and human life.

The phylum Echinodermata unites about 5,000 species of marine benthic animals, mostly mobile, less commonly attached to the bottom by a stalk. Representatives of the phylum Echinodermata are characterized by the following features: 1. Their body exhibits radial pentamerous symmetry, although their ancestors were bilaterally symmetrical. 2. In their integuments, echinoderms possess a skeleton made of calcareous plates, along with spinous outgrowths whose tips emerge on the body surface. 3. Echinoderms are coelomate animals, with the coelom differentiating into A number of systems, including the ambulacral system. 4. Echinoderms possess a Circulatory system; respiratory Organs are poorly developed or absent; there are no specialized excretory organs. 5. The Nervous system is primitive and partially embedded in the Skin integument. 6. Echinoderms are dioecious, and egg Cleavage is radial. 7. Development occurs with metamorphosis: the dipleurula larva serves as an intermediate stage. The phylum Echinodermata is divided

into two subphyla: Pelmatozoa and Eleutherozoa. The first subphylum includes mostly fossil classes and one extant Class, Crinoidea (sea lilies). The second subphylum includes 4 extant classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins), and Holothuroidea (sea cucumbers). This subphylum also includes the extinct class Ophiocystiodea.

The Class Asteroidea unites over 1,700 species of mobile predatory animals inhabiting various depths in seawater with a salinity concentration of at least 30%. The body of a sea star resembles a five-pointed star, in which a central disc and five rays (arms) are distinguished. In some species, the number of arms may be greater (from 6 to 30). The body features lines running from the center to the tips of the arms—radii, and lines terminating at the edge of the disc between adjacent arms—interradii. The body of sea stars is flattened. In the center, on the oral side, is the Mouth, while on the opposite, aboral side, is the anus. Sea stars move using ambulacral tube feet located at the bottom of the ambulacral groove. The echinoderm coelom consists of several sections, forming multiple cavity systems. A calcareous skeleton develops in the subcutaneous Connective Tissue. In each arm, There are two rows of ambulacral plates connected in pairs by Muscles. Spines, needles, and other structures emerge from the skeletal plates onto the body surface. In addition, movable pedicellariae may also form. The Digestive System begins with the mouth on the oral side, which is surrounded by a soft ring-shaped lip. The mouth connects via a short Esophagus to a pouch-like Stomach. From The Stomach, five pairs of long canals with lateral outgrowths—hepatic caeca—branch out into the coelom of the arms.

The nervous system is primitive and lies predominantly in the superficial epithelium. The main part of the nervous system consists of a circumoral nerve ring and five pairs of radial nerves lying at the bottom of the ambulacral groove. In addition to this ectoneural (oral) nervous system, sea stars have two deep systems—the hyponeural and the aboral nervous systems. The Sense Organs include the ambulacral tube feet, as well as five short tentacles at the tips of the arms. At the base of each tentacle lies a single eyespot. The perhaemal, or pseudohaemal, system consists of circumoral and radial canals branching into the walls of the ambulacral tube feet. The circulatory system is built on a radial plan. From the oral ring, radial Blood lacunae extend into the arms (located in the partitions of the perhaemal canals) along with an axial organ that runs toward the aboral side of the body. The Pericardium also belongs to the circulatory system; its pulsation drives the fluid within the blood lacunae of the axial organ. Sea stars breathe with the help of dermal branchiae (skin gills). Oxygen passes directly through the skin into the coelomic fluid.

The ambulacral system begins with the madreporite, which leads into the ampulla and further into the stone canal, whose oral end connects with the circumoral ring canal. The ambulacral radial canals originate from the ring canal and extend to the tip of each arm along the bottom of the ambulacral grooves. Numerous branches extend laterally from the radial canals, passing through pores between the ambulacral plates and connecting with the ambulacral tube feet. Most sea stars are dioecious. The Gonads are located in pairs in each arm in the form of clusters. A separate duct extends from each gonad, opening in the interradius. The number of eggs spawned can reach 200 million. Fertilization is external.

The body of a sea lily consists of a small cup-shaped trunk from which five long arms depart, branching extensively at the base. Sea lilies have a well-developed skeleton in the form of calcareous plates. The animals attach to the substrate using the terminal segment of the stalk, which expands to form a disc with hooks, ROOT-like cirri, or movable cirri. The arms of sea lilies feature a well-developed skeleton formed by individual cylindrical segments—ossicles—movably connected by elastic ligaments and Muscle fibers. The coelom is predominantly spongy, while free sections are located near the oral side of the body, in the arms, and in the lower part of the calyx near the central plate, where the coelomic cavity is divided by septa into five chambers, thus being called the five-chambered organ.

The ambulacral system consists of a circumoral ring and five radial canals that expand toward the branching arms. From 5 to 150 stone canals extend from the ring canal. There is no madreporite; it is replaced by the entire oral surface, which is pierced by so-called water pores. The digestive system begins with the mouth on the central disc. The mouth connects to the esophagus, which transitions into a long endodermal intestine. The intestine forms several loops and opens via the anus in one of the interradii. The perhaemal system is underdeveloped: it consists of five thin radial canals stretching along the arms beneath the radial canals of the ambulacral system and ending blindly. The circulatory system consists of a circumoral plexus of lacunae that transition into the lacunae of the intestinal walls and the axial coelomic sinus. Radial lacunae located in the walls of the arm cords and pinnules also extend from the circumoral ring. Sea lilies breathe through skeletal-free areas of the skin and ambulacral tentacles. The nervous system is characterized by a high Development of the apical part, which consists of a central nerve capsule inside the five-chambered organ and five radial nerves piercing all segments of the arms and pinnules. The ectoneural part of the nervous system consists of a circumpharyngeal ring and five radial nerve cords extending along the ambulacral grooves of the arms. The hyponeural nervous system has a similar structure. All sea lilies are dioecious. Gonads form within the arms. Fertilization is predominantly external. A doliolaria larva develops from the fertilized egg; it lacks both mouth and anal openings, but is instead "adorned" with an apical tuft of cilia and five ciliated bands. The next intermediate stage is the pentacrinoid stage, after which the individual transforms into a young sea lily.

The Class Holothuroidea, or sea cucumbers, numbers 1,100 species, of which 8 species are recorded in the Black Sea. Holothurians are relatively large animals, with an average size of 10–40 cm. Their body coloration is predominantly brown, off-white, and gray. The body is elongated from the oral to the aboral pole. Most holothurians resemble thick worms. At the anterior end of the body is the mouth, surrounded by a ring of tentacles, while at the opposite end is the anus. The body surface is covered by non-ciliated epithelium. The skeleton is heavily reduced and represented only by microscopic calcareous bodies in the dermis (cutis) and a circumpharyngeal ring comprising five large radial and many interradial sclerites. Holothurians have a well-developed musculature formed by a layer of circular muscles and a layer of longitudinal muscles.

The ambulacral system has a structure similar to that of other echinoderms. The main organ of locomotion is the ambulacral tube feet equipped with suckers. The digestive system begins with the mouth, surrounded by 8–30 tentacles—modified ambulacral tube feet. The mouth connects to the Pharynx, which passes through the calcareous ring into a relatively narrow esophagus that sometimes features an expansion at its end. The esophagus transitions into a long midgut that extends to the posterior end, forms several loops, and opens via the anus, expanding just before it to form a cloaca. The perhaemal system in holothurians consists solely of radial canals.

The circulatory system consists of a lacunar circumpharyngeal ring, five pairs of radial lacunae, A large number of lacunae in the connective tissue, and a system of vessels in the intestinal walls, of which the dorsal and ventral vessels are the largest. Respiration in holothurians occurs through the skin and ambulacral tentacles, and in some species, through respiratory trees (water Lungs). The nervous system of holothurians is located beneath the skin. The ectoneural nervous system is the best developed. The apical nervous system is absent, and in the hyponeural system, the ring part has undergone reduction. Among sense organs, statocysts are developed in deep-sea holothurians. Some holothurians possess photoreceptor Cells at the base of their tentacles. Most holothurians are dioecious. There is only a single gonad without a genital cord. The genital duct opens in the dorsal interradius near the anterior end of the body. Fertilization in most sea cucumbers is external. The Development of holothurians whose eggs are poor in yolk proceeds through several larval stages: ciliated blastula, or gastrula, and dipleurula.