CHAPTER XII.

OF ECHINODERMATA.

311. As we ascend the scale of Animal life, we meet with such a rapid advance in complexity of structure, that it is no longer possible to acquaint one's self with any organism by microscopic examination of it as a whole; and the dissection or analysis which becomes necessary, in order that each separate part may be studied in detail, belongs rather to the Comparative Anatomist than to the ordinary Microscopist. This is especially the case with the Echinus (sea-urchin), Asterias (star-fish), and other members of the class Echinodermata; since even a general account of their complex organization would be quite foreign to the purpose of this work; whilst there are certain parts of their structure, which furnish microscopic objects of such beauty and interest that they cannot by any means be passed by; besides which, recent observations on their embryonic forms have revealed a most unexpected order of facts, the extension and verification of which will be of the greatest service to science,—a service that can only be effectually rendered by well-directed Microscopic research in fitting localities.

312. It is in the structure of that calcareous skeleton, which probably exists, under some form or other, in every member of this class, that the Microscopist finds most to interest him. This attains its highest development in the Echinida; in which it forms a box-like shell, or "test," composed of numerous polygonal plates jointed to each other with great exactness, and beset on its external surface with "spines," which may have the form of prickles of no great length, or may be stout club-shaped bodies, or, again, may be very long and slender rods. The intimate structure of the shell is everywhere the same; for it is composed of a network, which consists of carbonate of lime with a very small quantity of animal matter as a basis, and which extends in every direction (i. e. in thickness, as well as in length and breadth), its areola or interspaces freely communicating with each other (Fig. 234). These "areola," and the solid structure which surrounds them, may bear an extremely variable propor

FIG. 234.

tion, one to the other; so that in two masses of equal size, the one or the other may greatly predominate; and the texture may have either a remarkable lightness and porosity, if the network be a very open one, like that of Fig. 235, or may possess a considerable degree of compactness if the solid portion be strengthened. Generally speaking, the different layers of this network, which are connected together by pillars that pass from one to the other in a di

Section of Shell of Echinus, showing the rection perpendicular to their plane,

calcareous network of which it is composed:-a a, portions of a deeper layer.

FIG. 235.

are so arranged that the perforations in one shall correspond to the intermediate solid structure in the next; and their transparency is such, that when we are examining a section thin enough to contain only two or three such layers, it is easy, by properly "focussing" the Microscope, to bring either one of them into distinct view. From this very simple but very beautiful arrangement, it comes to pass that the plates of which the entire "test" is made up, possess a very considerable degree of strength, notwithstanding that their porousness is such, that if a portion of a fractured edge, or any

Transverse Section of the medullary other part from which the investing

membrane has been removed, be laid upon fluid of almost any description, this will be rapidly sucked up into its substance. A very beautiful example of the same kind of calcareous skeleton, having a

by boiling in caustic potass; and the appearance of one of the five segments of which it is composed, when thus prepared, is shown in Fig. 236.

313. The most beautiful display of this reticulated structure, however, is shown in the structure of the "spines" of Echinus, Cidaris, &c.; in which it is combined with solid ribs or pillars, disposed in such a manner as to increase the strength of these organs; a regular and elaborate pattern being formed by their intermixture, which shows considerable variety in different species. When we make a thin transverse section of almost any spine belonging to the genus Echinus (the small spines of our British species, however, being exceptional in this respect), we are at once made aware of the existence of a number of concentric layers, arranged in a manner that strongly reminds us of the concentric rings of an Exogenous tree (Fig. 167). The number of these layers is extremely variable; depending not merely upon the age of the spine, but (as will presently appear) upon the part of its length from which the section happens to be taken. The centre is usually occupied by a very open network (Fig. 235); and this is bounded by a row of transparent spaces (like those at a a, b b', e c', Fig. 237), which, on a cursory inspection, might be supposed to be void spaces, but which on a closer examination are found to be the sections of solid ribs or pillars, which run in the direction of the length of the spine, and form the exterior of every layer. Their solidity becomes very obvious, when we either examine a section of a spine whose substance is pervaded (as often happens) with a coloring matter of some

Portion of transverse section of Spine of Acrocladia mammillata.

depth, or when we look at a very thin section by the "blackground" illumination. Around the innermost circle of these solid pillars, there is another layer of the calcareous network, which again is surrounded by another circle of solid pillars; and this arrangement may be repeated many times, as shown in Fig. 237, the outermost row of pillars forming the projecting ribs that are very commonly to be distinguished on the surface of the spine. Around the cup-shaped base of the spine is a membrane

which is continuous with that covering the surface of the shell, and which serves not merely to hold down the cup upon the tubercle over which it works, but also, by its contractility, to move the spine in any required direction. This membrane is probably continued onwards over the whole surface of the spine, although it cannot be clearly traced to any distance from the base; and the new formations may be presumed to take place in its substance. Each new formation completely ensheaths the old; not merely surrounding the part previously formed, but also projecting considerably beyond it; and thus it happens that the number of layers shown in a transverse section, will depend in part upon the place of the section. For if it cross near the base, it will traverse every one of the successive layers from the very commencement; whilst, if it cross near the apex, it will traverse only the single layer of the last growth, notwithstanding that, in the club-shaped spines, this terminal portion may be of considerably larger diameter than the basal; and in any intermediate part of the spine, so many layers will be traversed as have been formed since the spine first attained that length. The basal portion of the spine is enveloped in a reticulation of a very close texture, without concentric layers; forming the cup or socket which works over the tubercle of the shell. The combination of elegance of pattern with richness of coloring, renders well-prepared specimens of these spines among the most beautiful objects that the Microscopist can anywhere meet with. The large spines of the various species of the genus Acrocladia furnish sections most remarkable for size and elaborateness as well as for depth of color (in which last point, however, the deep purple spines of Echinus lividus are pre-eminent); but for exquisite neatness of pattern, there are no spines that can approach those of Echinometra heteropora and E. lucunter. The spines of Heliocidaris variolaris are also remarkable for their beauty. No succession of concentric layers is seen in the spines of the British Echini, probably because (according to the opinion of the late Sir J. G. Dalyell) these spines are cast off and renewed every year; each new formation thus going to make an entire spine, instead of making an addition to that previously existing. Most curious indications are sometimes afforded by sections of Echinus-spines, of an extraordinary power of reparation inherent in these bodies. For irregularities are often seen in the transverse sections, which can be accounted for in no other way, than by supposing the spines to have received an injury when the irregular part was at the exterior, and to have had its loss of substance supplied by the growth of new tissue, over which the subsequent layers have been formed as usual. And sometimes a peculiar ring may be seen upon the surface of a spine, which indicates the place of a complete fracture, all beyond it being a new growth, whose unconformableness to the older or basal portion is clearly shown by a longitudinal section.

314. The spines of Cidaris present a marked departure from the plan of structure exhibited in Echinus; for not only are they destitute of concentric layers, but the calcareous network which forms their principal substance, is ensheathed in a solid calcareous cylinder perforated with tubules, which seems to take the place of the separate pillars of the Echini. This is usually found to close in the spine at its tip also; and thus it would appear that the entire spine must be formed at once, since no addition could be made either to its length or to its diameter, save on the outside of this sheath, where it is never to be found. The sheath itself often rises up in prominent points or ridges on the surface of these spines; thus giving them a character by which they may be distinguished from those of Echini. The slender, almost filamentary spines of Spatangus (Fig. 238), and the innumerable minute hair-like processes attached to the shell of Clypeaster, are composed of the like regularly reticulated sub

stance; and many of these are very beautiful objects for the lower powers of the Microscope, when examined by reflected light, and laid upon a black ground, without any further preparation. It is interesting also to find that the same structure presents itself in the curious Pedicellaria (forceps-like bodies mounted on long stalks), which are found on the surface of many Echinida, and the nature of which has been a source of much perplexity to Naturalists, some maintaining that they are parasites, whilst others consider them as proper appendages of the Echinus itself. 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 of their being truly appendages to it, as observation of their actions in the living state would indicate. Another example of the same structure is found in the peculiar system of plates which surrounds the interior of the oral orifice of the shell, and which gives support to the five teeth that may often be seen projecting externally through that orifice; the whole forming what is known as the "lantern of Aristotle." The