ANNELIDA. 199 



a long scries of minute ganglia joined by connecting filaments ; of 

 these, about twenty-four are situated along the ventral surface of 

 the body. The anterior pair, or that immediately beneath the 

 oesophagus, is larger than the rest, forming a minute heart-shaped 

 mass, which is united, by a delicate nervous collar embracing the 

 gullet, with two small nodules of neurine situated upon the dorsal 

 aspect of the mouth. The two minute ganglia last mentioned 

 form that portion of the nervous system most intimately connected 

 with sensation ; for, while the nervous filaments given off from the 

 abdominal ganglia are distributed to the muscular integuments of 

 the body, the nerves which issue from the supra-oesophageal pair 

 supply the oral sucker, where the organs of the senses are situated. 

 In all the homogangliata, indeed, it is exclusively from this ce- 

 phalic pair of ganglia that the nerves appropriated to the instru- 

 ments of the senses are derived, and we shall therefore not hesitate 

 in the following pages to apply to this part of the nervous system 

 of the articulata the name of brain ; considering it to be strictly 

 analogous, in function at least, with the cerebral masses of more 

 highly organized beings. 



When we regard the minute size of these, as yet rudimentary 

 nervous centres, we cannot, however, expect to find them asso- 

 ciated 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 ffirudimdte we 

 have, in addition, distinctly formed organs of vision, exhibiting, 

 indeed, the utmost simplicity of structure, but nevertheless cor- 

 responding in the perfection of their developement with the con- 

 dition of the cerebral masses in relation with them. 



(241.) 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 suck- 

 ing surface of the oral disc ; a position evidently calculated to ren- 

 der them efficient agents in detecting the presence of food. The 

 structure of these simple eyes, according to Professor M tiller,* 

 does not as yet present any apparatus of transparent lenses adapted 



* Annales des Sciences Nat. vol. xxii. 



