Fig. 96. - A, The masticatory apparatus of an Echinoid (Toxopneustes lividus), viewed from above, with part of the alimentary canal attached to it: a OEsophagus; b Heart, with the sand-canal (c) in a groove on one side ; d The summit of the masticatory apparatus, with some of the muscles (e) of the same. B, Minute structure of one of the plates of the test of an Echinus (greatly magnified), showing the calcified areolar tissue. C, The masticatory apparatus of Sphaerechinus esculentus, viewed from the inside and laterally, as seen in place: f f Peristomial margin of the corona; g g Two of the radiating ambulacral vessels, with their rows of ampullae.

The mouth conducts by a pharynx and a tortuous oesophagus to a stomach, opening into a convoluted intestine, which winds round the interior of the shell, and terminates in a distinct anus. The mouth is always situated at the base of the test, and may be central, subcentral, or altogether excentric in position. The anus varies considerably in its position, being usually situated within the apical disc, and surrounded by the genital and ocular plates, when the test is said to be "regular." Sometimes, however, the anal aperture is without the apical disc, and is removed to some distance from the genital plates, when the test is said to be "irregular." In this last case, the anus, instead of being apical, is marginal or submarginal. The convolutions of the alimentary canal are attached to the interior of the test by a delicate mesentery; the surface of which, as well as that of the lining-membrane of the shell, is richly ciliated, and subserves the purposes of respiration.

The proper blood-vascular system (fig. 96 A, b) consists of a central fusiform, contractile vesicle, or heart. This gives off one vessel which forms a ring round the intestine near the anus, and another which passes downwards, and forms a circle round the gullet, above the "circular canal" of the ambulacral system. From the anal vessel proceed five arterial branches, which run along the ambulacral spaces, and return their blood by five branches, which run alongside of them in an opposite direction. This system of vessels is not always present, and its true nature is doubtful. High authorities regard it as rather comparable to the " pseudohaemal" system of the Annelides, than to the blood-system of the higher animals; while eminent observers maintain that the so-called heart is really of a glandular nature.

The nervous system consists of a ganglionated circular cord, which surrounds the gullet below, or superficial to, the " circular canal" of the ambulacral system, and which sends five branches along the ambulacral spaces, in company with the radiating ambulacral canals.

The process of respiration is carried on partly by arborescent gill-like organs placed round the mouth, which are of the nature of greatly developed tube-feet, and which are not universally present; partly by the tube-feet and their secondary vesicles in general; and partly by the vascular lining of the test and the mesentery. The sea-water is admitted to the body-cavity principally through the "madreporiform tubercle," only a portion of the area of this being occupied by the stone-canal; though, as previously remarked, recent observations would go to show that this view is incorrect.

The sexes are distinct in all the Echinoidea, and the reproductive organs are in the form of five membranous sacs, which occupy the inter-ambulacral areas, and open on the exterior by means of the apertures in the genital plates. In the " irregular Echinoids (such as the " Heart-urchins") there are only four genital glands, and therefore only four genital plates in the apical disc.

As regards their development, most of the Echinoids pass through a metamorphosis, as spoken of previously in treating of the development of the class. In these cases the larva is so unlike the adult animal that it was originally described as a distinct animal under the name of Pluteus, from its resemblance to a painter's easel (fig. 92). The larva exhibits bilateral symmetry, and is furnished with provisional organs in the shape of ciliated epaulettes, a skeleton of calcareous rods, and an alimentary canal. The adult Echinoid is developed out of a portion of its substance only; and the rest of the larva is absorbed or thrown off. In some Echinoids, on the other hand, as we have seen, the process of development is direct, and there is no "Pluteus" stage, but the young animal is produced viviparously, and simply requires to grow to be converted into the adult.

The typical Sea-urchins are divided into the two great groups of the "Irregular" and "Regular" Echinoids (or the Echinoi-dea exocyclica and Echinoidea endocyclica). The Irregular Echinoids have the anus situated outside the apical disc, marginal or submarginal in position, and have only four genital plates. They are also mostly destitute of a masticatory apparatus; and are generally of an oblong, pentagonal, heart-shaped, or dis-coidal figure (as in the common "Heart-urchins" and "Cake-urchins"). The "Regular" Echinoids, on the other hand, have the anus placed at the summit of the test, surrounded by the genital disc; the test is almost always circular or spheroidal ; and the mouth is armed with a complicated masticatory apparatus.

Another singular group is that of the Echinothuridae, in which the test is "regular," but the plates of both the ambulacral and inter-ambulacral areas are imbricated and overlap one another, rendering the test quite flexible. The existing genera, Asthe-nosoma (or Calveria) and Phormosoma, and the Cretaceous genus Echinothuria belong to this group.

A fourth group of the Echinoids is that of the Perischoechi-nidae, which is not only extinct, but is wholly confined to the Palaeozoic period. In all these ancient forms there is the peculiarity that the test consists of more than twenty rows of plates; there being a multiplication of either the inter-ambulacral or the ambulacral plates, though there are still only five inter-ambulacral and five ambulacral areas. Thus in Archaeociaaris, Palaechinus, Lepidechinus, and Eocidaris, the ambulacral areas agree with those of the recent Urchins in being composed of only two rows of plates; whilst there are from three to eight or more rows of plates in each inter-ambulacral area. On the other hand, in Melonites and Oligoporus, the ambulacral areas consist, respectively, of ten and four rows of plates. In some of the Perischoechinidae the plates of the test are joined by their edges, as in the common living Urchins; but in others (e. g, Lepidechinus) the plates overlap in an imbricating manner, as in the recent Echinothuridae, and the test thus becomes flexible.