SANGUISUGA. 



upon the sucking surface of the oral disc ; a position evidently 

 calculated to render them efficient agents in detecting the 

 presence of food. The structure of these simple eyes does not 

 as yet present any apparatus of transparent lenses adapted 

 to collect or concentrate the rays of light, but each ocellus or 

 visual speck would seem to be merely an expansion of the 

 terminal extremity of a nerve derived immediately from the 

 brain, spread out beneath a kind of cornea formed by the del- 

 icate and transparent cuticle ; beh'ind this is a layer of black 

 pigment, to which naturally the dark color of each ocular point 

 is due. 



Leeches, like the generality of the Annelida, are hermaph- 

 rodite, every one possessing two complete systems of gener- 

 ative organs, one subservient to the impregnation, the other 

 to the production of the ova ; nevertheless, these animals are 

 not self-impregnating, but the congress of two individuals is 

 essential to fecundity. 



Commencing with the male organs, it is not surprising to 

 find the testes divided into numerous distinct masses, or rather 

 repeated again and again, in conformity with a law to which 

 allusion has already been had. The glands which appar- 

 ently secrete the seminal fluid are about eighteen in number, 

 arranged in pairs upon the floor of the visceral cavity. Along 

 the external edge of each series there runs a common canal, 

 or vas deferens, which receives the secretion furnished by all 

 the testicular masses placed upon the same side of the mesian 

 line, and conveys it to a receptacle where it accumulates. The 

 two reservoirs or vesiculce seminales communicate with a mus- 

 cular bulb situated at the root of the penis. The penis itself 

 is frequently found protruded from the body after death ; it is 

 a slender filament, which communicates by its origin with the 

 contractile bulb, and when retracted is lodged in a muscular 

 sheath. The male apparatus is thus complete in all its parts ; 

 the fecundating secretion derived from the double row of 

 testes is collected by the two vasa deferentia, and lodged in 

 the receptacles ; it is thence conveyed into the muscular cavity 

 situated at the root of the male organ of excitement, through 

 which it is ultimately ejected. 



The ovigerous or female sexual organs of the leech are 

 more simple in their structure than those which constitute the 

 male system ; they open externally by a small orifice situated 

 immediately behind the aperture from which the penis is pro- 



10 



