The tegumentary tissues, like all others, are produced by the metamorphosis of the pe riplast of the protomorphic or indifferent tissue from which they take their origin, the endoplasts, to all appearance, taking but little share in the metamorphic processes. The chemical metamorphosis of the periplast may be either into horny, chitinous, calcareous, or cellulose matter ; in form it may become fibrous, laminated, vacuolated, bony, prisma tic, &c.
As a general rule, the endoplasts tend to disappear,pari passu, with the metamorphosis in form and composition of the periplast; but the differences presented by different tissues in this respect have given rise to the esta blishment of a distinction between what is called the process of conversion and that of excretion. For instance, in the development of a hair or of' a nail, the elements of the pro tomorphic layer evidently pass, as such, into the perfect substance of these organs; the periplast simply becoming horny, and the endo plasts remaining for a long while, or even always, visible in the cornified tissue. This is therefore a process of "conversion" of the protomorphic tissue. On the other hand, the chitinous coat of the lower Annulosa and the shells of the lamellibranchiate and gasteropod Mollusks arise in a totally different manner. The elements of the protomorphic layer do not pass into them entire, but they are formed, like the cuticula of a plant, or like the den tine and enamel of the teeth, by the successive outgrowth of layers of the outer portion of the periplast. No endoplasts, therefore, are ever found in them, and there is no conversion of the protomorphic tissue, but a process of excretion.* At first sight this distinction would appear to be very decided, and likely to afford a good ground for the formation of definite sub divisions of the integumentary organs into classes. Unfortunately, it is often difficult in practice to assure oneself in what way a given tegumentary organ has been formed. While the presence of endoplasts in a meta morphosed tissue is good evidence of its having been developed by conversion, their absence is no proof that the tissue has been developed by excretion ; inasmuch as it rnay simply be due to their very early disappear ance. In fact, if any one affirm that the shell of
a Unio or of a Crustacean, notwithstanding the impossibility of detecting endoplasts in its youngest laminm, is in reality formed by the successive apposition of entire layers of the protomorphic tissue, in which the endoplasts disappear so early that they cannot be de tected, it would be very difficult absolutely to disprove the assertion, though we might ask for evidence of its truth. Disbelieving in the doctrine of the special vital activity of the endoplasts, I confess the question does not seem to me to be of much importance, and I have only enlarged upon the subject because great vveight has by high authorities been laid upon these distinctions. It appears to me that the processes of conversion and of excre tion grade one into the other, and that no real subdivisions can be based upon the oc currence of either to the exclusion of the other. I will, however, take care to indicate what appear to me to be dear instances of each. I shall now proceed to consider the histological structure of the integuments of animals in the following order : — 1. Hydroid and Actinoid Polypes and Beroidm. 2. An nulosa, including the Worms and Echinoderms. 3. Mollusca, including the Ascidians and Po lyzoa. 4. Vertebrata.
1. Hydroid and Actinoid polypes.—In these animals the integument consists either of a simple cellular and vacuolated ecderon, or the outer layer of this is developed into a structureless coat, which may become thick ened by repeated additions, and thus attain considerable dimensions. In the common Campanularia, for instance, the outer wall of the bud from which a polype is to arise consists, at first, of a mass of indifferent tissue. As development proceeds, the outer portion of the mass is converted into a structure less membrane, which becomes detached from the body of the polype through its whole extent, and constitutes the future cell, the subjacent ecderon taking on the ordinary cellular structure. On the pedicle the same process goes on to a less extent, the struc tureless layer becoming separated only at intervals, so that the pedicle acquires a ringed appearance.