180 SEXUAL DIMORPHISM 



Elasmobranchs. Darwin states that the "claspers" of 

 Elasmobranchs serve to retain the female. It is known 

 from one observation in the aquarium that they are really 

 intromittent organs, inserted in copulation into the cloaca 

 and oviducts of the female, but whether their function in 

 this position is rather to hold the female, or to convey 

 the semen into the female organs, is open to question. 

 The claspers are great enlargements and modifications of the 

 hinder parts of the pelvic fins, and such modification is 

 entirely wanting in the females. They are only fully de- 

 veloped in mature males. Owing to the complicated mus- 

 cular and skeletal apparatus they contain, it seems evident 

 that they exercise a powerful dilating action on the oviducts 

 and cloaca, and although the necessity of such muscular 

 power is difficult to understand, the fact of its exertion 

 agrees well with the view that the claspers owe their evolu- 

 tion to the functional activity of the muscles concerned in 

 the act of copulation. 



In the Eays the males have special spines which are absent 

 in the females. They are situated on the dorsal side, some 

 at the outer margin of the head, but more on the pectoral 

 fins, a little distance from the extremities of these. They 

 are rather long, simply bent or hooked, and very sharp. They 

 are set in somewhat irregular longitudinal rows, and can be 

 erected or depressed at the will of the fish. When depressed, 

 they lie close to the skin, and may easily escape observation. 

 These spines are stated by Darwin to be only developed 

 during the breeding season, but I do not know on what 

 authority. 



In Raia clavata the teeth of the adult male are sharp- 

 pointed and directed backwards, while those of the female 

 are broad and flat, forming a pavement. In the young male 

 the teeth are flat, like those of the female. Darwin suggests 

 that the sharp teeth of the males may be used for fighting or 



