224 



ANNELIDA. 



which are independent, to a certain extent, of the general circulation, 

 and that opposite to 



each membranous bag ^S- 109- 



a portion of the fluid 

 contained in the lateral 

 vessel (a) is given off 

 through the muscular 

 tube (>) (which thus re- 

 sembles a pulmonary 

 heart), and, after being 

 distributed over the 

 walls of the supposed 

 respiratory vesicle, and 

 in this manner exposed 

 to the influence of oxy- 

 gen, the blood returns 

 into the general circu- 

 lation*. 



(584.) The nervous 

 system of the Leech 

 (fig. 110, Tc} consists of 

 a long series of minute 

 ganglia joined by con- 

 necting 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 fila- 

 ments 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 cephalic pair of ganglia that the nerves appropriated to the 

 instruments of the senses are derived ; and we shall therefore not hesi- 

 tate 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 analo- 

 gous, in function at least, with the cerebral masses of more highly 

 organized beings. 



* As will be seen further on, the so-called "pulmonary hearts" of M. Duges are 

 regarded by Dr. Williams as ovario-uterine organs. 



Respiratory organs of the Leech, according to M. Duges : 

 a, lateral vessel; 6, pulmonary heart; c, dorso-lateral 

 branch ; d, latero-abdominal vessel ; e, afferent pulmonary 

 branch; /, respiratory sacculus. 



