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PHYLUM CHORDATA 



181 



development of these gill-filaments in the embryos of some vivi- 

 parous 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 



m.brn 



m.brn 



FlG. 856. Side view of head of embryo 

 of ScyUium canicula, with the 

 rudiments of the gills on the first and 

 second branchial arches, eye, eye ; 

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

 mix. nasal sac. (After Sedgwick.) 



FlG. 807. Side view of the head of Seyllium 

 canicula at a somewhat later stage. The 

 gill-filaments have increased in number and 

 are present on the mandibular arch. ami. 

 angle of the jaw ; hy. hyoid : m. brn. mid- 

 brain : nas. nasal sac ; spir. spiracle. (After 

 Sedgwick.) 



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 protovertebras : 

 these, the 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, waging 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. 



Nearly all are marine : some ascend rivers : a very few live 

 habitually in fresh water. The only deep-water Elasmobranch 



1 In a species of Trygon a number of the villi of the uterus project into 

 the pharynx of the fnetus through the spiracles, and nourishment is probably- 

 received by this means. 



VOL. II M* 



