INVERTEBRATE ZOOLOGY IN THREE VOLUMES - BOOK 3 - H.I. Shcherbak - 1997

DEUTEROSTOMIA

PHYLUM ECHINODERMATA

SUBPHYLUM ECHINOZOA

CLASS ECHINOIDEA

SUBCLASS IRREGULARIA

The body of irregular sea urchins is strongly flattened in the oral-aboral direction and varies in shape. The anal opening shifts along the interradius from the center of the aboral pole to the margin, and sometimes even to the oral pole. The interradius where the anus is located is called posterior, and the opposite one is anterior. Other radii and interradii are arranged more or less symmetrically on the sides of the plane passing through the anterior radius and posterior interradius, which makes the animal's body bilaterally symmetrical (Fig. 208).

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Fig. 208. Diagram of the shell Structure in irregular sea urchins — dorsal (a) and ventral (b) views:

1, 2 — interambulacral and ambulacral plates; 3 — petaloid; 4 — madreporite; 5 — tubercles for spine attachment; 6 — peristomial membrane; 7 — scraper; 8 — sternal plates; 9 — anus

The ambulacral tube feet on the upper part of the body have lost their locomotory function and transformed into respiratory Organs—Skin gills.

The subclass Irregularia includes four orders. Representatives of two of them—Holectypoida and Cassiduloida—flourished during the Jurassic and Cretaceous periods of the Mesozoic era, after which they nearly went extinct; only two species of the first order and about 30 species of the second have survived to the present day. Let us examine the other two orders: Clypeasteroida and Spatangoida.

Order Clypeasteroida. The body of clypeasteroids is strongly flattened and only slightly thickened in a few species. Its outline is nearly circular; sometimes there are perforations along the margins, as in Echinodiscus biperforata, or notches, as in Rotulus orbiculus (Fig. 209). On the upper surface of the body, There is a characteristic five-petaled flower-like pattern formed by widened ambulacral series (petaloids). The test spines are short and delicate; they cover the urchin's body so densely that they create the impression of a velvet coating. Most Representatives of the order inhabit tropical seas on soft substrates.

Fig. 209. Order Clypeasteroida (tests):

a — Rotulus orbiculus; b — Echinodiscus biperforatus

Order Spatangoida. In many Spatangoida, there is a depression at the outer end of the anterior radius, giving their body a Heart-shaped form, though oval, bottle-shaped, and other forms also occur among them. In A number of species, not only the anal opening but also the Mouth moves forward along the anterior radius, sometimes reaching the margin of the body, which further enhances the bilateral Symmetry of such urchins (Fig. 210).

Fig. 210. Order Spatangoida:

a — Pourtalesia jeffreysi; b — Echinosygra paradoxa; c — diagram of the urchin's position in the sediment

Unlike clypeasteroids, spatangoids have a convex upper body surface bearing four petaloids. The internal anatomy of spatangoids also features several differences: they lack Aristotle's lantern, and one, two, or three Gonads are reduced.

Representatives of the order inhabit soft substrates, burrowing into them to a depth of two or more centimeters (up to 15–20 cm) to search for food within this layer or acquire it by other means. When the urchin is embedded in the sediment, it remains connected to the surface by a vertical burrow (tube) whose walls are reinforced with mucous secretions produced by the urchin. Through this tube, vigorous Movements of the spines generate a Water current that supplies oxygen to the skin gills located on the petaloids. The vertical burrow is also used for food acquisition: the anterior tube feet extend significantly, and with the help of their thickened, sticky tips, the required amount of food is quickly gathered on the sediment surface around the burrow opening. The tube feet are then retracted into the burrow, and the food is passed by the spines to the mouth. At the same time, the posterior tube feet extend backward into the posterior tube, where excrement is channeled.