SANGUISUGA. 



the lateral vessel is given off through the muscular tube, which 

 thus resembles a pulmonary heart, and after being distributed 

 over the walls of the respiratory vesicle, and in this manner 

 exposed to the influence of oxygen, the blood returns into the 

 general circulation. 



The nervous system of the leech consists of a long series 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 oesopha- 

 gus, 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 inti- 

 mately connected with sensation ; for while the nervous fila- 

 ments given off from the abdominal ganglia are distributed to 

 the muscular integuments of the body, the nerves which issue 

 from the supra-cssophageal pair supply the oral sucker, where 

 the organs of the senses are situated. In all the Homogan- 

 gliata, indeed, it is exclusively from this cephalic pair of gan- 

 glia that the nerves appropriated to the instruments of the 

 senses are derived, and therefore this part of the nervous sys- 

 tem of the Articulata is not improperly called the brain, and 

 considered by most naturalists' to be strictly analogous, in 

 function at least, with the cerebral masses of more highly 

 organized beings. 



When the minute size of these as yet rudimentary nervous 

 centres are regarded, it cannot be expected 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 animals, but in the Hirudinidce there are, in 

 addition, distinctly formed organs of vision, exhibiting, indeed, 

 the utmost simplicity of structure, but nevertheless correspond- 

 ing in the perfection of their development with the condition 

 of the cerebral masses in relation with them. 



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 

 semicircular row of black points, situated above the mouth, 



9 



