THE ELASMOBRANCH FISHES 



27 



occurrences exceed in lengtli many times the diameter of the body of the cell. 

 From the possession of great numbers of these processes expanded into a com- 

 plex web, the extreme black of a form like Torpedo occidentalis results. 



The golden cells or lipophores (Ip., fig. 30) (Xanthophores) containing the 

 fatty pigment, lipochrome, are beautifully shown in the young of Heterodon- 

 tiis. Here, with the brown melanophores, they give patches of a warm glossy 

 yellow of remarkable ])eauty. In Cephaloscyllium (fig. 1) also, multitudes of 

 lipophores produce the sulphur spots so characteristic of this form. In Rhino- 

 don (fig. 3) the lipophores associated with reddish brown melanophores form 

 the great orange spots or color patches. 



In most of the Elasmobranchs, ex- 

 cepting the deep-sea types, pigment 

 cells are absent from the venter. The 

 metallic white here results from the 

 presence of guanin, a waste product of 

 metabolism, which impregnates the 

 cells (leucophores) ventrally as do the 

 pigment granules dorsally. The guanin 

 granules although present are not visi- 

 ble dorsally, for in this location they 

 are obscured by the melanin granules. 

 Ventrally they are very numerous, and 

 have much to do with the production 

 of the light color. Contributing also to 

 the formation of a light-colored venter 

 is a certain concentration of tissues 

 known as argentium. In this concentra- 

 tion the underlying tissues, through the deposition of calcic prisms, become 

 so compact as to form a highly reflecting surface to which the silvery sheen 

 characteristic of a fresh specimen is partly due. 



The function of pigment has often been thought to be the protection of the 

 more delicate underlying tissues against the rays of the sun. While this per- 

 haps holds in general, such protection from the rays of the sun would not be 

 necessary for those forms like Spinax or Etmopteriis which inhabit the pro- 

 found depths into which the light of the sun never penetrates. And it is the 

 more singular that in such deep-sea Elasmobranchs pigmentation is not con- 

 fined to the dorsum but is distributed over the ventral part of the body as well. 

 It has been suggested that at depths at which the rays of the sun are unknown 

 it may still be that pigmentation is in some way correlated with light; for per- 

 vading even the greatest depths there is present phosphorescent light, the 

 source of which being diffuse would not result in pigment on the back alone 

 but on the sides and venter as well. But the cause of pigmentation in deep-sea 

 sharks is as yet not understood. It seems not improbable that the pigmentation 

 is correlated not so much with light as with the lower temperature. 



Fig. 30. Chroniatophores of young Hetero- 

 donfiis francisci. 



Ip., golden cells containing granules ; 

 pg., melanophore filled with brown pig- 

 ment. 



