432 FISHES CHAP. 



there is a hyoidean hemibranch supplied with venous blood from 

 the ventral aorta. The gill- filaments are attached throughout 

 their length to the interbranchial septa. There is an optic 

 chiasma. An air-bladder is not developed. The intestine has a 

 spiral valve, and there is a cloaca. The gonoducts in both sexes 

 are derived from the kidney system. The ova are large, few in 

 number, and enclosed in horny egg-cases, and they are fertilised 

 before extrusion. ' The segmentation is meroblastic, and the 

 embryo is furnished with long external gills. 



The Elasniobranchs are for the most part active predaceous 

 Fishes, living at different depths in the sea, from the surface to 

 nearly a thousand fathoms, and ranging from mid-ocean to the 

 shallower waters round the coasts in almost every part of the 

 world. Although typically marine, they sometimes ascend rivers 

 beyond the reach of tides, and a few are permanent inhabitants 

 of fresh water. They are most abundant in tropical and sub- 

 tropical areas, where they also attain their greatest size, and are 

 numerous in temperate regions, but there are some species which 

 are typically Arctic. None of them are small, and some of the 

 Sharks are the largest of living Fishes. All are carnivorous, but 

 so diversified is their food that in different species it may range 

 from other Fishes of no mean size to Molluscs, Crustaceans and 

 other Invertebrates, or even to plankton. In their breeding habits 

 the Sharks and Dog-Fishes present many interesting features. 

 Unlike the generality of Fishes, the eggs are fertilised internally 

 as a sequel to the copulation of the sexes. For this purpose the 

 males are furnished with special intromittent organs, the myxo- 

 pterygia or so-called claspers, which are developed as modifica- 

 tions of the hinder portions of the pelvic fins.^ Each clasper is 

 supported by an internal skeleton, consisting of several cartilages 

 derived from the radialia of the fins, and is traversed along its 

 inner aspect by a groove. When sexual congress takes place the 

 claspers are thrust through the cloaca of the female into the ovi- 

 ducal orifices, and in some instances it is probable that they are 

 retained in this position by hook-like denticles developed at 

 their free extremities. The seminal fluid then flows along these 

 conduits into the oviducts, in the upper portions of which it meets 

 and impregnates the eggs. After fertilisation the egg is enclosed 

 in a dark brown horny egg-case, secreted by the oviducal gland. 

 ' Den Danske Ingolf-Ex^pedition, ii. No. 2, Copenhagen, 1898. 



