172 



ZOOLOGY 



SECT. 



approaches completion, their bases alone persisting, to give rise 

 to the permanent gills. The great development of these gill- 

 filaments in the embryos of some viviparous forms suggests that 

 in addition to their respiratory functions they may also serve as 

 organs for the absorption of nutrient fluids secreted by the villi of 

 the uterine wall. 1 The fins, both paired and unpaired, appear as 

 longitudinal ridges of the ectoderm enclosing mesoderm. In some 

 Elasmobranchs the paired fins are at first represented on each side 

 by a continuous ridge or fold, which only subsequently becomes 

 divided into anterior and posterior portions the rudiments 

 respectively of the pectoral and pelvic fins. Into these folds 

 penetrate a series of buds from the prot overt ebras ; these, the- 



m.Jir-n. 



m.brn 





Gi 



pir -r 



FIG. 797. Side view of head of embryo 

 of Scy Ilium canicula, with ^the 

 rudiments of the gills on the first and 

 second branchial arches, eye, eye ; 

 in. brn. mid -brain ; mnd. mandible ; 

 nas. nasal sac. (After Sedgwick.) 



FIG. 70S. Side view of the head of Scyllium 

 canicula at a somewhat later stage. The 

 gills have increased in number and are present 

 on the mandibular arch. anr/. angle of the 

 jaw ; hj/. hyoid ; m. brn. mid-brain ; ,w*. nasal 

 sac ; spir. spiracle. (After Sedgwick.) 



muscle-buds, give rise to the fin-muscles ; at first, from their mode 

 of origin, they present a metameric arrangement, but this is in 

 great measure lost during development. 



Ethology and Distribution. The habits of the active, fierce, 

 and voracious Sharks, which live in the surface-waters of the sea, 

 making war on all and sundry, contrast strongly with those of 

 the more sluggish Rays, which live habitually on the bottom, 

 usually in shallow water, and feed chiefly on Crustaceans and 

 Molluscs, with the addition of such small Fishes as they can 

 capture. As a group, the Elasmobranchs, more particularly the 

 Sharks, are distinguished by their muscular strength, the activity 

 of their movements, and also by the acuteness of their senses of 

 sight and smell. 



The only deep-water Elasmobranch known is a species of Ray, 

 which extends to a depth of over 600 fathoms. 



1 In a species of Trygon the nutrient secretion is stated to pass into the 

 enteric canal through the spiracles. 



