TEGUMENTARY ORGANS 423 



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 fibril- 

 lated uncalcified 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 peculiar bundles 

 and elastic fibres which are met with in fully formed connective 

 tissue. 



In Paludina, according to Leydig, the pigment 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 him- 

 self points out, they frequently 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. According to H. Miiller,^ who has recently made 

 some careful investigations on this subject, there lie beneath the 

 cellular ecderon in these animals : 1st, a fibrous layer, usually colour- 

 less, 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 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 we trace it inwards, 

 developing its elastic element 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 superimposed horizontal strata, tied together by perpendi- 

 cular columns, which penetrate the interspaces of the bundles, and 

 spread out into the irregular connective tissue on the deep and super- 

 ficial 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 



^ Bericht, &c. Zeitschrift fur Wiss. Zoologie. lSi53' 



