(507). The tubercles upon the external surface of the shell of the Echini support a corresponding number of long spines, which, as well as the apparatus of suckers, are employed as locomotive agents. These spines vary materially in their form and proportionate size, and even in their internal structure and mode of growth, as may be readily seen by a comparison of different species. Thus, in the flattened forms of Scu-tellce and allied genera, they are so minute as to require the employment of a microscope for their investigation; in Echinus esculentus (fig. 85) they are sharp, and almost of equal length over the entire surface of the animal; while in the specimen represented in the annexed figure (fig. 95), the shell of which we have already examined when divested of these appendages, the length of the spines that are articulated upon the large tubercular plates fully equals the transverse diameter of the body of the creature, and in some cases they are even found much more largely developed. Every spine, examined separately, is seen to be united with the tubercle upon which it is placed by an apparatus of muscular and ligamentous bands, forming a kind of ball-and-socket joint, allowing of a considerable extent of motion. The structure of this articulation is exhibited in fig. 94, 2. The large tubercle (a) supports upon its apex a smaller rounded and polished eminence, perforated in the centre by a deep depression; and the bottom of the moveable spine (c) is terminated by a smooth hemispherical cavity accurately fitted to the projecting tubercle, so that the two form complete articular surfaces. The bonds of union connecting the spine with the shell are of two kinds: in the first place, there is a stout ligament (a, c), extending from the little pit seen upon the centre of the tubercle, to a corresponding depression visible upon the articular surface of the spine, resembling very accurately the round ligament found in the hip-joint, and obviously a provision for the prevention of dislocation.

Cidaris.

Fig. 95. Cidaris.

* Cyclopaedia of Anat. and Phys., art. "Echinodermata".

(508). Moreover, the whole joint is enclosed in a muscular capsule, composed of longitudinal fibres (b b) arising from the circumference of each tubercle, and inserted all around the root of the spine: these fibres, therefore, which must, in fact, be regarded as merely derived from the general irritable skin that clothes the shell externally, are the agents which, acting immediately on the spine, produce all the movements whereof it is capable.

(509). The next thing to be accounted for in the history of these elaborately-constructed animals is the growth of the spines themselves: these, as we have already seen, are completely detached from the rest of the shell, to which they are secured only by the central ligament and by the muscular capsule enclosing their base. To account, therefore, for the production of organs so completely insulated as the spines appear to be, especially when we consider that there is no vascular communication between them and the body of the Echinus, would appear to be a matter of some difficulty; and in fact, had we not already seen, in the Polyps, the amazing facility with which calcareous matter was secreted by the living textures of those animals, it would be almost impossible to conceive by what process their growth was effected. On examining one of these appendages, taken from a species wherein they are largely developed, when fresh, before its parts have become dry, every portion of its surface is seen to be invested with a thin coat of soft membrane, derived from that which covers and secretes the whole shell, whereof indeed the muscular capsule enclosing its articulation with the tubercle is only a thickened portion.

(510). The living covering of the spine, therefore, like the crust that invests the cortical Polyps, is the secreting organ provided for its growth, depositing the earthy particles separated from the waters of the ocean, layer after layer, upon its outer surface, so as to form a succession of concentric laminae, of which the outer one is always the last formed. The calcareous matter thus deposited has, more or less completely, a crystallized appearance; and on a transverse section of the organ being made, and the surface polished by grinding, the whole process of its formation is at once rendered evident. Such sections, indeed, form extremely beautiful and interesting subjects for microscopical examination, as nothing can exceed the minute accuracy and mathematical precision with which each particle of every layer composing them appears to have been deposited in its proper place: in fact, if the zootomist would fully appreciate the minuter details connected with their organization, it is only by the employment of the microscope that he will arrive at adequate ideas concerning them; for it is not in the number and variety of the pieces entering into the composition of the skeleton of one of these animals, the extraordinary apparatus of prehensile suckers with which they are furnished, or the singular locomotive spines upon the exterior of the shell, that he will find the most remarkable features of the history of the Echini; it is only by a minute examination of the intimate structure of each of these parts that the perfection of the mechanism conspicuous throughout can be properly understood.

(511). The calcareous pieces surrounding the mouth of the Echinus are not so immoveably consolidated as those composing the rest of the shell, but, on the contrary, admit of considerable movement, whereby the prehension of food is materially facilitated. The mouth itself (fig. 94, l) is a simple orifice, through which the points of five sharp teeth are seen to protrude. These teeth obviously perform the office of incisors, and from their sharpness and extreme density are well calculated to break the hard substances usually employed as food. The points of such incisor teeth, although of enamel-like hardness, would nevertheless be speedily worn away by the constant attrition to which they are necessarily subjected, were there not some provision made to ensure their perpetual renewal; like the incisor teeth of rodent quadrupeds, they are therefore continually growing, and are thus always preserved sharp and fit for use. In order to allow of such an arrangement, as well as to provide for the movements of the teeth, jaws are provided, that are situated in the interior of the shell; and these jaws, from their great complexity and unique structure, form, perhaps, the most admirable masticating apparatus met with in the whole animal kingdom; we must therefore entreat the patience of the student while we describe at some length the parts connected therewith.