200 ANNELIDA. 



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

 sual 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 delicate and transparent 

 cuticle : behind this is a layer of black pigment, to which the dark 

 colour of each ocular point is due. 



(242.) Leeches, like the generality of the Annelida, are herma- 

 phrodite, every one possessing two complete systems of generative 

 organs, one subservient to the impregnation, the other to the produc- 

 tion 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, we are not surprised to find 

 the testes divided into numerous distinct masses, or rather repeated 

 again and again in conformity with a law to which we have already 

 alluded ( 229). The glands which apparently secrete the semi- 

 nal fluid are about eighteen in number (Jig. 80, 2, e, /), arranged 

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

 edge of each series there runs a common canal, or vas defer ens 9 

 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 (d), where it accumulates. The two reservoirs, or 

 vesicula seminales, if we may so call them, (d, d,) communicate 

 with a muscular bulb (c) situated at the root of the penis. The 

 penis itself (a) is frequently found protruded from the body after 

 death ; it is a slender tubular filament, which communicates by its 

 origin with the contractile bulb (c), and, when retracted, is lodged 

 in a muscular sheath (5). 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 (d, d) ; it is thence conveyed into the muscular cavity 

 (c) situated at the root of the male organ of excitement, through 

 which it is ultimately ejected. 



(243.) 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 protruded, the two 

 openings being separated by the intervention of about five of the 

 ventral rings of the body. The vulva, or external canal, leads into 

 a pear-shaped membranous bag (Jig. 80, 2, g), which is usually, 

 but improperly, named the uterus. Appended to the bottom of 

 this organ is a convoluted canal (^), which communicates with two 



