030 ECHINODEKMATA are usually narrower than the interambulacra, have near their outer edge small shield-like spaces, umbones, bounded by a more or less elevated wall, and perforated by pairs of small orifices or pores for the protrusion of the feet or pedicels, each pair of pores corresponding to one of the pore plates the primitive ossicles which commonly unite to form the ambulacral plates. The ambulacra are either homogeneous, i.e., composed of similar elements gradually diminishing in size towards the poles of the test, or (as in the Spatangoida and most of the Clypeastroida) are hetero geneous, having the upper portion petaloid in shape, and the lower with pures scattered in areas not always confined to the ambulacral plates, or arranged in ramifying fascise. In the Spatangoida the anterior unpaired ambulacrum is commonly obsolete (see iig. 5). In the Oolitic genus FIG. 5. Spatangus purpureus. Dysaster, the two postero-lateral ambulacra, forming the livium, are ssparate from the rest, and converge over the anal opening ; while the three anterior, the trivium, unite at the apical disk (see fig. 8). The growth of the urchin in length is effected by the formation of new plates at the apical end of the corona, and in breadth by additions to the margins of the plates. On the surface of the plates are tubercles of different sizes, each with a knob or elevation, sometimes crenulated, by which the acetabulum of the spine is attached (see figs. 4 and 5). The presence or absence in the tubercle of a central perforation for the passage of a ligament for the spine is an important dis tinguishing character in various groups of fossil echini. The spines in the young state are ciliated; like the plates of the test they are composed of a calcareous network, and are inter penetrated and covered by the perisome, which contains the muscular fibres by which they are moved. They are short in the Clypeastroida and Spatangoida, and of various lengths in the Echinoida, and offer a considerable diversity of form and ornamentation. Dr Gray (Ann. of Nat. Hist., i. p. 414) mentions the discovery in Sicily of the fragment of a spine of an echinus, the circumference of which was nearly 1| inches, and tho length more than 8 inches. In Porocidaris purpu- rata, a deep-sea form, the spines are paddle-shaped, and very flat, and are serrate on the edges ; in Coelopleunis the long curved spines resemble the antennae of certain beetles. Scattered over the surface of the test, and more especially on the oral membrane, are the pedicellarige. generally regarded as pecu- I- 1 IMS j /? in harly modified spines ; these, when well developed (fig. G), consist of a long flexible stem, fur open; &, closed. (After Gegeubaur.) nished at the summit with a forceps of three pincers or prongs, which snap together, and seize firm hold of any object that comes in their way. They serve for the removal from the neighbourhood of the shell of dirt of all kinds, and apparently also for defence. Caheria fenestrata has pedicellaria? with four valves. In some Spatangoid genera the corona bears symmetrical bands of minute tubercles with attached spines, the stmitw or fas- doles, distinguished, according to their position with re spect to the anus or to the apical or the marginal termi nations of the petaloid ambulacra, as sub-anal, drcum-anal, intrapetalous, and peripetalous. The spines of the semita- have a thick integumentary covering, and except at the enlarged apex, are closely studded vith cilia. Love n has shown the occurrence, in all Echinidea but Cidaris, of another kind of appendages of the test, possibly sensory organs, to which, he has given the name of sphceridia. These are button-like, spheroidal bodies, seldom above i^th inch in length, furnished with a .short stalk, and normally articulated with small projecting tubercles on the plates of the ambulacra and peristome. Sometimes they become concealed by a layer of the test, in vhich there remains only a fine external fissure. At the summit or apical pole of the test is a space occupied by the ocular and genital plates, which in the Echinoida (Enclocyclica) encircle the anus with its anal plates. The five genital plates, vhich are opposite the interambulacra, or interradial in position, are perforated by apertures for the exit of the reproductive products. In the Clypeastroida and Spatangoida (Exo- cyclica), in which the anus is eccentric, and exterior to the apical disk, one of the genital plates is usually hnperforate (fig. 7). The five ocular plates are situated radially, crown ing the apical ends of the ambu lacra ; on the surface of each is a depression, having a pore for an ocellus or eye-spot. Always, except in the Clypeastrida 1 , the right antero lateral genital plate, or, in other words, that situated to the right of the anterior ambu lacrum of the trivium, is larger than the others, and bears the convex, perforated madreporic tubercle or madreporite. In the Upper surface (a), showing four Clypeastroida this is most fre- J^SSffi^SS quently extended over the other mouth and posterior border, apical plates. The hinder genital plate, with apparently one exception, is wanting in the Spatangidae, its place being occupied by the madreporite. The Palajozic Echiniclea differ from the more modern forms by the greater number of perforations of their ocular and genital plates. At the base of the test is the mouth with its buccal membrane and plates. The species Leskia mirabilis (the type of the sub-family Leskiadce, family Spatangida-) has botli mouth and anus closed by converging triangular valves. In the Echinoida and Clypeastroida the mouth is central in position, and pro vided with teeth ; in the Spatangoida it is eccentric and edentulous. The teeth resemble those of Kodents in form, and are arranged in hard wedge-shaped sockets or alveoli, which by their union form a pentagonal cone. As the outer substance of the tooth is harder than the inner, it is less readily worn away, and thus always presents a sharp edge. Each alveolus is composed of two halves united in the middle line, and each half, again, consists of a superior and inferior portion. The alveoli are inter-radial in position, or opposite the interambulacra. They are connected by transverse muscular fibres, and alternate with superiorly placed, thickish, radial structures, the rot idee orfalces, which, in the Echinoida, bear each a bifurcated piece, the radius. This skeletal mouth-apparatus is commonly known as FIG. 7. Echinocyanius sillus.
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