Although there is nothing like interstitial growth in the shell or spines of the Echinus, yet both are progressively enlarged by the addition of new matter. The polygonal plates of which the shell is composed are separated from each other by a membrane that passes into every suture ; and the margins of each plate appear to receive periodical additions, by calcareous deposit in the substance of this membrane. In this manner the globular form of the entire shell is preserved, whilst it undergoes progressive enlargement ; new plates being added, as they may be required, round the anal orifice of the shell (Agassiz). There can be little doubt that the spines are, in like manner, periodically augmented in diameter by suc cessive formations or acts of growth, which take place in the investing membrane ; and a longitudinal section of the spine makes it evident that these additions not only sur round the preceding deposits from the base upwards, but pass considerably beyond them, thus adding to the length of the spine. The consequence is, that a transverse section taken near the base of the spine will exhibit all the layers of which it is made up, each layer being narrow, and the central medulla small. A section taken at about the middle of the length may very probably not cut across the original spine nor the older layers, which do not reach so far ; and a section taken across the spine near its apex will only traverse the one or two layers last formed. Nevertheless, in many species, the spine is larger at that part than near its base ; but the large size is due to the great ex pansion of the medullary centre, which is composed of a very loose calcareous reti culation.
The structure of the shell of the Echinits is repeated in that of the three genera which may be regarded as the types of the principal subdivisions of the order Echinida, — namely, Cidaris, Clypeaster, and Spatangus : there can be no reasonable doubt, therefore, that it is universal throughout the group. The spines, however, of Cidaris, present a marked vari ation from the plan of structure exhibited in Echinus ; for they are usually nearly cylin drical in form, destitute of concentric layers, and composed of a calcareous reticulation en veloped in a cylinder of a solid, apparently homogeneous substance, chiefly calcareous, which rises 'up in ridges upon the exterior. Hence it would appear that, like endogenous trees, whatever additions these spines may receive in length, they can receive little or none in diameter. The slender, almost fila mentary species of the Spatangace, and the innumerable minute hair-like processes at tached to the shell of the Clypeasteridce, are composed of a like regular reticulated tissue ; many of these are extremely beautiful objects when examined with the microscope without any preparation. It is interesting also to remark, that the same structure presents itself in the Pedicellaria, which are found upon the surface of many Echinida, and which have been so great a source of perplexity to naturalists. The complete conformity which exists between the structure of their skeleton, and that of the animal to which they are attached, would seem to remove all reasonable doubt that they are truly appendages to it ; as their actions also would indicate.
The same structure presents itself in the calcareous plates which form the less perfect skeletons of the Asteriada, and also in their spines, when these (as in the large Goniaster equestris) are furnished with a calcareous frame-work, and are not mere projections of the hard integument. It is also met with
in the family Ophiuricke, which forms, in some respects, the transition to the Crinoidal group ; but the calcareous skeleton is here generally subordinate to the firm and almost horny integument. In the Crinoidea, on the other hand, the calcareous skeleton is highly developed, and its structure is extremely characteristic. This is well displayed in the recent Pentaerinus Caput Medusa', the stem and branches of which are made up of a calcareous net-work, closely resembling that of the shell of the Echinus. There is ex hibited, moreover, in a transverse section of the stem of Pentaerinus, as in the spines of Echinus, a certain regular pattern, which results from the varying dimensions of the areolm in different parts. This pattern, formed hy the extension of five pairs of rays (strongly reminding us of the medullary rays of plants) from the centre towards the cir cumference, is frequently well preserved in the fossilized stems of Pentacrini, and varies in different species sufficiently to serve as a distinctive character. In the round-stemmed Encrinites, a transverse section of the joints exhibits a simple concentric arrangement.
It only remains for us to notice the order Holothuridce, in which, as is well known, the calcareous skeleton of the other Echinoder mata is reduced to its most rudimentary con dition; never forming a complete and con nected framework, but only showing itself in detached pieces, the disposition of which is extremely variable. In the typical Holo thuria, there are five solid calcareous plates around the mouth, in which the calcareous reticulation is very characteristically seen. Each of the tentacula, also, has a small cal careous disk at its extremity, which presents a sort of rude sketch of the beautiful struc ture of the rosette that supports the ambu lacral suckers of the Echinus.
There can be no reasonable doubt that this peculiar arrangement is universal throughout the group, since it has been detected in cha racteristic examples of every one of its prin cipal subdivisions. And, consequently, as no similar calcareous reticulation is found in the internal or external skeleton of any other animal, even the minutest fragment which distinctly presents this structure inay be re ferred with certainty to an Echinoderm. And this structure is perfectly preserved, even after the substance has been infiltrated with calcareous matter in the act of fossilization, and has become so completely mineralised, that the disposition to rhomboidal fracture rnakes it difficult to obtain a section in any other direction than that of the plane of cleavage. As already remarked, the elemen tary structure is essentially- the same every where ; so that it might not be possible to determine from a very minute fragment whe ther it formed part of the shell of an Echinus, Cidaris, or Spatangus,—a portion of the frame work of an Asterias, Ophiura, or Holothuria, —or entered into the composition of the stem of an Enerinite. But where any regular pat tern is displayed, this is frequently sufficient to distinguish the genus, or even the species, to which the fragment belonged. This is certainly the case in regard to the spines of Cidarites and the stems of Pentaerinites ; and will probably be found no less true in other instances, when these beautiful structures shall have been more extensively investigated.