INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOA (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA), OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION DEUTEROSTOMES (DEUTEROSTOMIA)

PHYLUM ECHINODERMATA

CLASS CRINOIDEA (CRINOIDS)

The most ancient group of Echinoderms. There are about 4,000 fossil species and more than 620 modern species known, distributed mainly in the tropical Regions of the Indian and Pacific Oceans, although some are also found in Antarctic and Arctic waters. The size of sea lilies ranges from a few centimeters to 2.5 m. The Class name refers to the resemblance of these animals to flowers, which is further emphasized by their bright coloration. Some crinoids remain attached to the substrate throughout their lives, some can detach for a time and move short distances, while others lead a free-living lifestyle, swimming or crawling and periodically attaching to the substrate.

Crinoids differ from all other echinoderms in that their body faces the oral side upward into the Water Column, with the Mouth and anus located there. In their Structure, radial Symmetry predominates over bilateral symmetry.

Structure. The body consists of a small, more or less goblet-shaped trunk from which five long rays, or arms, extend. These arms fork at their base and often branch repeatedly, with the number of branches reaching up to 200 (Fig. 432).

Fig. 432. The sea lily Heliometra glacialis (adapted from Baranova): 1 - pinnules; 2 - rays, or arms; 3 - cirri

Integument. A single-layered epithelial covering is present only in young individuals, whereas in adults all external skeletal plates are bare. In areas lacking a Skeleton, Connective Tissue lies beneath the epithelium.

The Skeleton of the oral side is underdeveloped and represented by small calcareous sclerites, whereas the aboral goblet-shaped side is covered by a solid armor formed by the lower central plate and two to three whorls of five skeletal plates each. In some crinoids, the aboral skeleton is also significantly reduced, changing the shape of the trunk so that it becomes almost disk-shaped. In most sessile species, a stalk extends from the aboral side of the body, consisting of cylindrical or disk-shaped calcareous ossicles connected by Muscle fibers, which provides mobility to the stalk. Its length in modern species is 75-90 cm, while in fossil forms it reached 21 m. Some crinoids attach to the substrate by the terminal ossicle of the stalk, while others use numerous fine ROOT-like structures or mobile cirri. By means of cirri (which can number up to 100) with claws at the end, stalkless crinoids temporarily anchor to the bottom.

The arms of crinoids have a well-developed skeleton consisting of individual cylindrical ossicles, or vertebrae, and a weakly developed soft part that houses some Internal Organs. The arm vertebrae are connected by elastic ligaments and muscle fibers, providing exceptional flexibility and mobility. However, at certain points, The connection between ossicles occurs without Muscles, and autotomy (spontaneous shedding of a body part) frequently occurs at these junctions. Almost every vertebra has lateral branchlets—pinnules—articulated on the right and left, which share the same structure as the arms. On the inner side of the vertebrae and their branches, including the pinnules, There is a groove along which runs a radial ambulacral furrow covered with ciliated epithelium. The arms form a perfect feeding apparatus to supply the animals with food. In addition, using their arms, free-living crinoids can crawl or swim quite adeptly.

The body cavity of adult animals is largely filled with a network of connective tissue trabeculae lined with coelomic epithelium, making the coelom spongy. Free coelomic spaces remain near the oral side of the body, where the coelom extends into the arms, and in the lower part of the calyx near the central plate, where the coelomic cavity is divided by partitions into five chambers and is therefore called the five-chambered organ (Fig. 433). The coelom continues from the calyx into the stalk and cirri.

Fig. 433. Horizontal section of a crinoid calyx (adapted from Strelkov et al.):

1 - radial nerve cord; 2 - central nerve capsule; 3 - membrane of the five-chambered sinus;

4 - its chamber; 5 - muscles; 6 - axial organ; 7, 8 - central and radial plates of the aboral skeleton

The ambulacral system of crinoids differs from that of other echinoderms. They lack a madreporite. There are from 5 to 150 stone canals, all of which open with their free ends into the body cavity. The entire oral side of the body fully corresponds to the madreporite, being pierced by numerous so-called water pores that connect the coelom with the surrounding water. The radial canals branch in accordance with the branching of the arms, including the pinnules. Lateral canals depart from these radial canals and lead into two rows of ambulacral tentacles (podia) on the oral side of the rays and pinnules. The tentacles lack suckers and serve as respiratory and tactile organs.

The Digestive System begins with the mouth located in the center of the oral side. The mouth leads into the Esophagus, which continues into a long endodermal intestine lined with ciliated epithelium; it initially

extends toward the aboral pole, then makes one to four loops and turns back toward the oral side, where it opens via a short hindgut through an anus situated on a small protuberance located asymmetrically near the mouth (Fig. 434). Crinoids feed on numerous microscopic animal and plant organisms, as well as their remains, which are filtered from the water through the dense network of ambulacral tentacles on the arms and directed toward the mouth by the beating of cilia along the grooves.

Fig. 434. Oral disk of the crinoid Heliometra glacialis (adapted from Strelkov et al.):

1 - oral opening surrounded by papillae; 2 - ambulacral grooves; 3 - anal cone; 4 - first pair of pinnules;

5 - arms; 6 - rudimentary plates of the oral skeleton; 7 - water pores

Reproductive System. All crinoids are dioecious, though Sexual Dimorphism is absent. Gonads develop within the arms. Five genital cords branch in accordance with the branching of the arms. Their terminal branches extend into the pinnules, where they transform into blind-ending sacs; it is within these sacs that Germ Cells develop.

Reproduction. Male Gametes are released from the pinnules through specialized pores, whereas female gametes are released via ruptures in the pinnule walls. In many species, the sperm is released into the water first, which stimulates female individuals to shed their eggs. Fertilization is most commonly external. In cases of internal fertilization, spermatozoa actively penetrate the pinnules of the female.

A barrel-shaped larva—the doliolaria, bearing an apical tuft of cilia and five ciliated bands, and lacking both mouth and anus—develops from the fertilized egg. For two to three days it leads a planktonic existence, after which it settles to the bottom and attaches itself by its anterior end to the substrate or some hard object, and occasionally to the body of its parents. The larval body begins to elongate and differentiate into a stalk and a cup-shaped trunk, at the apex of which the mouth breaks through; the ciliated rings and apical tuft degenerate. Skeletal plates form within the mesoderm of both the cup and the stalk. Outgrowths appear along the margins of the cup and develop into arms. This stage is called the pentacrinoid stage due to its resemblance to stalked crinoids of the genus Pentacrinus (Fig. 435).

Fig. 435. Larvae of the crinoid Antedon rosacea (after Ivanova-Kazas):

a - doliolaria; b - pentacrinoid: 1 - continuous ciliated bands; 2 - semi-ring anterior ciliated band;

3 - attachment fossa; 4 - skeletal plates

Further development proceeds differently in sessile and free-living forms. In the former, new stalk segments continuously form, and cirri and additional branches develop, anchoring the stalk firmly to the substrate. In the latter, after several months of a sessile existence, the calyx spontaneously detaches, and the crinoid transitions to a free-living mode of life. Antarctic crinoids that exhibit parental care are particularly interesting; for instance, in species of the genus Phrixometra, embryos develop within brood chambers located in the genital pinnules of the females.

Crinoids play a role in the biological purification of marine water from organic pollutants. Their calcareous skeletons contribute to the composition of sedimentary rocks, such as limestone and marble. Fossil crinoids can serve as index fossils for determining the geological age of sedimentary strata.