502 



TEGUMENTARY ORGANS. 



crease in size by the external addition of new 

 concentric lamina?; the direction of the latter 

 not beingparallel with, or having any reference 

 to, that of the laminae of fibrous membrane 

 with which they so amalgamate; thus they 

 are not depositions from, but growths in the 

 membrane ; which growths, as they increase 

 in size, retain their primitive tendency to 

 assume a lenticular form." Following the 

 layers of the scale outwards, these isolated 

 calcareous deposits not only enlarge, but 

 ultimately become fused together, forming at 

 length either a continuous calcareous mass in 

 each layer, or presenting fissures which in 

 some cases traverse the original lenticular 

 calcareous deposits, in others are interstitial 

 to them. I think one cannot but be struck 

 with the complete analogy between the struc- 

 ture and mode of developement here described 

 and those which I have previously shown to 

 obtain in the calcified tegumentary organs of 

 the Mollusca and Crustacea. The ganoin 

 layer corresponds very closely with the " epi- 

 dermis " of the shell or test ; the middle 

 laminated calcified substance is formed by the 

 fusion of concentrically laminated concretions 

 deposited in a membranous matrix in the 

 Fish, the Mollusk, and the Crustacean alike ; 

 while the deep uncalcified layers of the scale 

 are represented by the " horny " laminae which 

 have escaped calcification in Haliotis or Unio, 

 and still more closely by the fibrillated un- 

 calcified layers of the Crustacean test. 



Structure of the enderon. The enderon of 

 the Invertebrata is usually entirely composed 

 of rudimentary connective tissue or of mere 

 indifferent tissue, consisting, in the latter case, 

 simply of a matrix with imbedded endoplasts, 

 while in the former it is produced into plates 

 and bands, never exhibiting, however, the pe- 

 culiar bundles and elastic fibres which are met 

 with in fully formed connective tissue. 



In Paludina, according to Leydig, the pig- 

 ment masses, which lie on the surface of the 

 ecderon, are connected by " clear large cells, 

 with a small parietal nucleus." From their 

 occurrence, wherever in the higher animals 

 connective tissue is found, Leydig calls them 

 '* Binde-substanz-zellen " " Connective tissue 

 cells ; " but, as he himself points out, they fre- 

 quently contain carbonate of lime, and their 

 relation is rather, like that of the similar cells 

 in Piscicola, to fat. 



A wonderful complication of structure is 

 attained by the skin of the Cephalopoda. Ac- 

 cording to H. Miiller *, who has recently made 

 some careful investigations on this subject, there 

 lie beneath the cellular ecderon in these ani- 

 mals : 1st, a fibrous layer, usually colourless, 

 but occasionally white and glittering. 2nd, the 

 layer with the chromatophora (vide inf.). 3rd, 

 beneath these a peculiar layer, which gives rise 

 to the colours produced by interference, the 

 metallic lustre, and intense whiteness of many 

 localities. It consists frequently of regular 

 plates, which evidently proceed from nucleated 

 cells. 4th, deeper still lie the larger bundles 



* Bericht, &c. Zeitschrift fur Wiss. Zoologic. 

 1853. 



of connective tissue, the muscles and the 

 vessels. 



In the Vertebrata, the superficial layer of 

 the enderon is similarly composed of indifferent 

 tissue, and of rudimentary connective tissue ; 

 the former passing gradually into the latter, as 



Fig. 319. j^ 



Enderon of the Skate. 



we trace it inwards, developing its elastic ele- 

 ment to a greater or less extent, and acquiring 

 a more or less distinctly fascicular arrangement 

 of its collagenous element. In the higher 

 Vertebrata, these bundles are usually disposed 

 as an irregularly felted mass ; but in Fishes 

 and Batrachia, they form regularly super- 

 imposed horizontal strata, tied together by 

 perpendicular columns, which penetrate the 

 interspaces of the bundles, and spread out 

 into the irregular connective tissue on the 

 deep and superficial surfaces of the stratified 

 mass (fig. 319. A). On the addition of acetic 

 acid, it is seen that the boundaries of the strata 

 are formed by irregular bands of elastic tissue, 

 in which the remains of the primitive endo- 

 plasts may be seen (as in fibro-cartilage), 

 whose strongest fibres are horizontal, though 

 they send out others irregularly in all direc- 

 tions. The perpendicular columns are likewise 

 composed of bundles of pale elastic fibres (fig. 

 319.B), and if the intersection of the horizontal 

 with the vertical divisions be carefully examined, 

 it is seen that the former are, as it were, given 

 off by the latter, which thus gradually break 

 up and thin out, terminating above and below 

 in the elastic fibres of the unstratified super- 

 ficial and deep layers. A horizontal section of 

 this portion of the enderon presents a very 

 peculiar appearance, the transparent vertical 

 columns looking like radiating spaces, as which 

 they were, in fact, at first described. 



Pigment of the enderon. The enderon 

 presents scattered masses of pigment, some- 

 times contained in cells and sometimes free, 

 in many Invertebrata (Annelids, Trematoda, 

 Echinoderms, Crustacea, Mollusca). In 

 other Invertebrata and in the higher Verte- 

 brata, the pigment is confined to the ecderon. 

 In Fishes and Reptiles, however, a well- 

 marked layer of pigment lies at the surface of 

 the enderon in the form of scattered granules 



