484 



TEGUMENTARY ORGANS. 



they are coincident; that soft membranous 

 separation therefore, which exists between 

 the two layers anteriorly, is far less developed 

 posteriorly ; and the soft continuation of the 

 scale which is flat anteriorly, is inflected pos- 

 teriorly ; the process of addition being other- 

 wise the same. Suppose, now, that each 

 detached calcareous centre of ossification as it 

 is added to the posterior margin of the scale, 

 instead of being flattened, were produced into 

 a spine as in the Rays, then it is perfectly clear 

 that instead of a cycloid scale, the result 

 would be a serrated ctenoid scale. And this 

 appears to be exactly what takes place in 

 the scales of the perch, according to Prof. 

 Williamson's description. 



From all this, 1 think, we arrive at Prof. 

 Williamson's conclusion, that fish-scales are 

 essentially tegumentary teeth ; that like the 

 latter organs, they result not from the calcifi- 

 cation of the cellular ecderon covering those 

 folds of the integument, upon which they are 

 developed and which correspond with the 

 dental pulp, but by a calcareous deposit taking 

 place beneath this, in what represents a deep 

 layer of the ecderon ; finally that it is, for the 

 present, an open question whether the deep 

 layers of all scales are produced by a con- 

 tinuation of this process, or whether in some 

 cases a deep truly enderonic structure may be 

 added to this superficial ecderonic constituent 

 to constitute the perfect scale. A process of 

 the latter kind would, at any rate, find its 

 parallel in the eventual union of the teeth of 

 many fishes with their jaws, and in that of 

 the plates of the chelonia with the vertebral 

 elements. 



3. Histology of the tegumentary organs. 

 Having thus arrived at a general idea of the 

 mode in which the various forms of integumen- 

 tary organs are produced from the primary 

 morphological constituents of every integu- 

 ment, we have now to consider their minute 

 histological elements and the mode in which 

 these proceed from the indifferent tissue of 

 which all organs are primarily composed. 



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, Sic. 



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, amfthe 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 may 

 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 lamina?, 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 weight 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 clear 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 Beroidae. 2. An- 

 nulosa, including the Worms and Echinoderms. 

 3. Mollusca, including the Ascidians and Po- 

 ly zoa. 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 



* Using the word in the sense of " growth out," 

 not in the common perverted signification of fluid 

 transudation and hardening. 



