NERVOUS SYSTEM AND SENSES OF THE LEECH. 225 



(585.) The splanchnic system in the Leech consists of three small 

 ganglia, situated in front of the brain, with which they are connected by 

 delicate nervous filaments. All three send branches to the parts around 

 the mouth and to the inferior surface of the alimentary apparatus*. 



(586.) When, however, we regard the minute size of these as yet 

 rudimentary nervous centres, we cannot expect to find them associated 

 with any very perfect apparatus of sensation. The oral sucker, indeed, 

 seems to possess a more delicate sense of touch than the rest of the 

 body, adapting it to examine the surface to which it is about to be 

 fixed ; and probably the Leech may enjoy, in some measure, perceptions 

 corresponding with those of taste and smell. These senses have been 

 found to exist in many of the animals we have already described ; but in 

 the Hirudinidce we have, in addition, distinctly-formed organs of vision, 

 exhibiting, it is true, the utmost simplicity of structure, but neverthe-^ 

 less corresponding, in the perfection of their development, with the con- 

 dition of the cerebral masses in relation with them. 



(587.) The eyes of the Leech are eight or ten in number, and are 

 easily detected, by the assistance of a lens, under the form of a semi- 

 circular row of black points, situated above the mouth, upon the sucking 

 surface of the oral disk, a position evidently calculated to render them 

 efficient agents in detecting the presence of food. The structure of 

 these simple eyes, according to Professor Miillerf, does not as yet pre- 

 sent any apparatus of transparent lenses adapted to collect or concen- 

 trate 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 delicate and transparent cuticle : behind this is a layer of black 

 pigment, to which the dark colour of each ocular point is due. 



(588.) Leeches, like the generality of the Annelida, are hermaphrodite, 

 every one possessing two complete systems of generative organs, one 

 subservient to 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. 



(589.) 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. The glands that apparently secrete the seminal fluid are 

 about eighteen in number (fig. 110, 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 deferens, which receives the secre- 

 tion furnished by all the testicular masses placed upon the same side of 

 the mesial line, and conveys it to a receptacle (d), where it accumulates. 

 The two reservoirs, or vesiculce seminales (d d), if we may so call them, 

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



* See Brandt und Eatzeburg, Med. Zool. tab. 29. t Ann. d. Sci. Nat. vol. xxii. 



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