404 TEGUMENTARY ORGANS 



relation to ordinary shell as a nail bears to a horny epidermis among 

 the higher animals. We know, in fact, that the nail, though to all 

 intents and purposes mere cornified epidermis, is at first an internal 

 structure, being covered over by the outer layers of the foetal epiderm. 

 A nail remaining so covered would correspond with the shell of Limax 

 or Sepia, while an ordinary nail represents that of Clausilia. Gegen- 

 baur, in fact, has shown that the shell of the latter mollusk commences 

 at first like that of Limax by the deposition of a layer of calcareous 

 particles in the midst of the cellular ecderon of the mantle beneath its 

 outer layer of cells. The shell of Limax goes no further than this 

 stage, while in Clausilia (and probably in Helix, &c.) it gradually 

 increases by addition to its under surface, and finally bursts through 

 the cellular investment which takes no share in its formation. It is 

 the same with Sepia. Here the internal shell, or sepiostaire, is com- 

 posed of two layers, a dorsal and a ventral ; the former, according to 

 Kolliker, is a thin membrane composed of slightly wav)', parallel, 

 somewhat dark fibrils 0'00i-2" broad, which frequently appear to be 

 composed of still more delicate fibrillae. So far as this membrane 

 corresponds with the ventral layer, it is covered on both surfaces by a 

 thin structureless lamina of carbonate of lime, which has a pearly 

 aspect on the ventral surface where it is not covered by the ventral 

 layer ; while it is granular on the dorsal surface, and on the ventral, 

 where it is covered by the proper ventral layer, presents ridges to 

 which the plates of the latter are attached. The thick ventral layer 

 of the sepiostaire is composed of lamellae set at a very oblique angle 

 to the dorsal layer, and united together by close-set partitions at right 

 angles to their surface. Acted upon by acid, this portion of the shell 

 leaves behind it a membranous skeleton of exactly the same form, 

 but presenting no further structure. 



Young embryos present merely a fibrous rudiment of the dorsal 

 layer. The ventral layer is formed by the successive deposit of 

 calcareous laminae inwards. When the first lamina has been formed, 

 a deposition of small cylindrical bodies take place upon its inner 

 surface. These increase, widen and become ramified at their extremities, 

 forming ramified columns. A second calcareous lamina is now 

 formed, connecting their ramified extremities, upon whose under 

 surface the like process takes place, and this is repeated until the 

 ventral layer has attained its full thickness. The ramified columns 

 are regularly transversely striated ; with the age of the shell additions 

 are continually made to their lateral dimensions until they coalesce 

 and constitute the septa of the perfect shell, upon which the stria; 

 remain visible. 



