Passing by many other points of interest in the Echinoidæ, the spines are seen to be attached to the test or shell by a ball and socket joint and well-arranged muscles, whereby the spines can be moved in any direction. The tubercles, however, do not cover the whole test, but are disposed chiefly in five broad zones extending from one pole to another. When a transverse section of a spine is examined by a medium power it is seen to be made up of a series of concentric and radiating layers (shown in [Plate XVIII]., Nos. 1 and 2), the centre being occupied by reticulated structure and structureless spots arranged at equal distances; these may be termed ribs or pillars. Passing towards the margin are other rows conveying the impression of a beautiful indented reticulated tissue. Many of the spines present no structure, while others exhibit a series of concentric rings of successive growth, which strongly remind one of the medullary rays of plants. When a vertical section of a spine is submitted to examination, it is seen to be composed of cones placed one above the other, the outer margin of each cone being formed by the series of pillars. In certain species of Echinus the number of cones is very considerable, while in others there are seldom more than one or two to be found; from these, transverse sections may when made show no concentric rings, only the external row of pillars.
The skeleton of echinoderms contains but a small amount of organic matter, as will be seen on dissolving out the calcareous portion in dilute nitric or hydrochloric acids. The residuum structure will appear to be meshes or areolæ, bounded by a substance having a fibrous appearance, intermingled with granulous matter; in fact, it bears a close resemblance to the areolar tissue of higher animals, and the test may be considered as formed, not by the consolidation of the cells of the ectoderm, as in the mollusc, but by the calcification of the fibro-areolar tissue of the endoderm. This calcification of a simple fibrous tissue by the deposit of a mineral substance, not in the meshes of areolæ but in intimate union with the organic basis, is a condition of much interest to the physiologist; it presents an example of a process which seems to have an important share in the formation and growth of bone, namely, in the progressive calcification of the fibrous tissue of the periosteum membrane covering of the bone.
The development of the sea-urchin from the fertilised egg first divides and then sub-divides, and in a short time the embryo issues forth with a small tuft of cilia, by means of which it swims off freely. The larvæ, in its full development, measures about one millimetre in diameter, and is a curious and remarkable creature.
The sub-kingdom Mollusca comprises some fifty thousand species, and fresh forms are being constantly discovered, the number of the aquatic genera being more than double that of the terrestrial species, for it matters not to what depth of ocean the dredge is let down, some new form is certain to be gathered. The Challenger expedition has enriched our knowledge of the deep-sea fauna to an enormous extent; so much so, that fifty volumes have already been published descriptive of animals brought to the surface. Nevertheless, we are told that the great coast lines of South America, Africa, Asia, and parts of Australia have been but imperfectly explored for smaller kinds of Mollusca.
Molluscs are soft-bodied, cold-blooded animals, without any internal skeleton, but this is compensated for by the external hardened shell, which at once serves the purpose of bones, and is a means of defence. These bodies are not divided into segments like those of worms and insects, but are enveloped in a muscular covering or skin, termed the mantle, the special function of which in most species is the formation and secretion of the shell. The foot, which serves the double purpose of locomotion and burrowing in the sand or rock, is an organ particularly characteristic of most molluscs. There are many departures from this rule, as, for instance, in the group Chitonidæ, where the shell takes the form of a series of eight adjacent plates; and in another, the Pholadidæ, there are one or more accessory pieces in addition to the two principal valves. Some are bivalved, others univalved, and concealed beneath the skin. All shells are mainly composed of carbonate of lime, with a small admixture of animal matter. Their microscopic examination reveals a great diversity of structure, as we shall presently see, and they are accordingly termed porcellaneous, nacreous, glassy, horny, and fibrous. Most molluscs have the power of repairing injuries to their shells; many exhibit an outer coat of animal matter, termed the peristracum, the special function of which is to preserve the shell from atmospheric and chemical action of the carbonic acid in the water in which they dwell.
The shells of gastropods are enlarged with the growth of the mollusc by the addition of fresh layers to the margin. In some species the periodic formation of spines occurs; a typical case will be found among Muricidæ. The varied colours of shells are due to glands situated on the margin of the mantle, and beneath the peristracum; occasionally the inner layer of porcellaneous shells is of a different colour to the outer, as, for example, in the helmet-shells (Cassis), much used by carvers of shell cameos. Light and warmth, as in the vegetable kingdom, are the great factors in the production of brilliant colours. In cold climates land snails bury themselves in winter time in the ground or beneath decaying vegetable matter, and in hot seasons they close up the aperture of the shells with a temporary lid, called an epiphragm. These exhibit great tenacity of life, as, for instance, in the Egyptian desert-snail, Helix desertorum. The reproductive system is in all cases effected by means of eggs. The ova are usually enclosed in capsules, and deposited in masses, and the number of eggs contained in the squid and the whelk have been stated to be thirty or forty thousand. The ova of molluscs may be gradually developed into the adult, or there may be a free-swimming ciliated larval stage, or a special larval form, as in the fresh-water mussel. Most are provided with a more or less distinct head; both cephalopods and gastropods are furnished with eyes. In land snails these are found placed on projecting stalks. In most cases the utility of molluscs far outweighs the injury occasioned by a few species, as, for instance, the Teredo, and the burrowing habits of the Pholas and Saxicava, compact marble having been found bored through by them.
Mr. J. Robertson wrote me in 1866:—“Having, while residing here (Brighton), opportunities of studying the Pholas dactylus, I have endeavoured during the last six months to discover how this mollusc makes its hole or crypt in the chalk—by a chemical solvent? by absorption? by ciliary currents? or by rotatory motions? My observations, dissections, and experiments set at rest controversy on this point. Between twenty and thirty of these creatures have been at work in lumps of chalk in sea water in a finger glass and a pan, at my window for the last three months. The Pholas dactylus makes its hole by grating the chalk with its rasp-like valves, licking it up when pulverised with its foot, forcing it up through its principal or branchial siphon, and squirting it out in oblong nodules. The crypt protects the Pholas from Conferveæ, often found growing parasitically not only outside the shell but even within the lips of the valves, thus preventing the action of the siphons. In the foot there is a spring, or style, which when removed is found to possess great elasticity, and this seems to be the mainspring of the motion of the Pholas.”
Fig. 367.—Hexabranchus.
I must pass by many groups and orders to more aberrant types, represented by the naked-gilled orders, Opisthobranchiata and Nudibranchiata. These gastropods constitute a large sub-order of extremely beautiful molluscs, remarkable in shape, and often brilliant in colour. The distinguishing character of these typical forms consists in the peculiar nature and situation of their breathing organs, which are exposed on the back of the animal or around the anterior part, and are not protected by the mantle. But the situation is varied, and the gills are sometimes placed on each side of the body, respiration being effected by the ciliated surface of the whole. For these and other reasons they have been placed in four groups. Nudibranchs are found in all parts of the world, and are most abundant in depths where the choicest seaweeds and corallines abound. Their fecundity is very great, as many as sixty thousand eggs being deposited by a single female at one time. They are eaten as a luxury where they most abound.