The members of this order - commonly known as Sea-urchins - are characterised by the possession of a subglobose, discoidal, or depressed body, encased in a "test" or shell, which is composed of numerous, usually immovably con-nected, calcareous plates. The intestine is convoluted, and there is a distinct anus. The sexes are distinct, and the larva is plutei-form, and has a calcareous skeleton. As regards their general anatomy, the "test" of the Echinoidea is composed of numerous calcareous plates,* which are generally firmly united to one another by their edges, in such a manner that the body of the animal is enclosed in an immovable box. In the singular Urchins, however, which constitute the family of the Echi-nothuridae, the plates of the test overlap one another in an imbricating manner, so that the shell becomes quite flexible; and the same is the case with some of the Palaeozoic Echinoids. In all living Sea-urchins, and in the great majority of the extinct forms, the test is composed of twenty meridional rows of plates, arranged in ten alternating zones (fig. 93, A), which typically pass from one pole of the shell to the other, and each of which is composed of two similar rows of plates. Five of these double rows are composed of large plates, which are not perforated by any apertures (fig. 93, A and B, a); the zones formed by these imperforate plates being termed the "inter - ambulacral areas." The other five double rows of plates alternate regularly with the former, and are termed the "ambulacral areas," or "poriferous zones." Each of these zones (fig. 93, A and B, i) is composed of two rows of small plates, which are perforated by minute apertures for the emission of the "ambulacral tubes," or "tube-feet." In one great group of the Echinoids, the ambulacral areas pass from the centre of the base of the shell to its summit, when they are said to be "perfect" (ambulacra perfecta) or "simple." In another great group the ambulacral areas are not thus continuous from pole to pole, but simply form a kind of rosette upon the upper surface of the shell. In these cases - as in the common Heart-urchins - the ambulacral zones are said to be "circumscript" (ambulacra circumscripta) or "petaloid" (fig. 93, D). Growth of the test is carried on by additions made to the edge of each individual plate, by means of an organised membrane which passes between the sutures where the plates come into contact with one another. The plates of the test are studded with large tubercles, which are more numerous on the inter-ambulacral areas than on the ambulacral (fig. 93, B). These tubercles carry spines (fig. 93, E, and fig. 94) used defensively and in locomotion, which are articulated to their apices by means of a sort of "universal" or "ball-and-socket" joint. Occasionally a small ligamentous band passes between the head of the tubercle and the centre of the concave articular surface of the spine, thus closely resembling the "round ligament" of the hip-joint of man. Besides the main rows of plates just described, forming the so-called "corona," other calcareous pieces go to make up the test of an Echinus. The mouth is surrounded by a coriaceous peristomial membrane, which contains a series of small calcareous pieces, known as the "oral plates;" whilst a corresponding series of "anal plates" is found in the membrane (fig. 93, C, p) surrounding the opposite termination of the alimentary canal. Surrounding the aperture of the anus at the summit of the test is the " apical disc," composed of the so-called genital and ocular plates (fig. 93, C). The "genital plates" are five large plates of a pentagonal form, each of which is perforated by the duct of an ovary or testis. One of the genital plates is larger than the others, and supports a spongy tubercle, perforated by many minute apertures, like the rose of a watering-pot, and termed the "madreporiform tubercle" (fig. 93, C, m). In some cases, this tubercle is not connected with one of the genital plates, but is placed in the centre of the apical disc. The genital plates occupy the summits of the inter-ambulacral areas. Wedged in between the genital plates, and occupying the summits of the ambulacral areas, are five smaller, heart-shaped, or pentagonal plates, known as the "ocular plates," each being perforated by a pore for the reception of an "ocellus" or "eye." (The existence of an eye-spot is denied by high authorities.)
* The skeleton of the Echinoids is composed of calcified areolar or connective tissue, the fibres of which enclose oval or rounded meshes (fig. 96, B), exhibiting under the microscope an exceedingly characteristic appearance.
Fig. 93. - Morphology of Echinoidea. A, Young specimen of Strongylocentrotus Drobachiensis, viewed from above. B, Small portion of the test of the same, magnified. C, Summit of the test of Echinus sphaera, magnified. D, Clypeaster subde-pressus, viewed from above, showing the petaloid ambulacra. E, Spine of Poroci-daris purpurata. F, Pedicellaria of Toxopneustes lividus. a a Ambulacral areas; i i Inter-ambulacral areas; g Genital plate; o Ocular plate; m Madreporiform tubercle; p Membrane surrounding the anus. (Figs. A, B, and D are after A. Agassiz.)