218 



ZOOLOGY. 



contrary, as in the leech, there are no articulated limbs, the 

 nervous ganglions are but little developed, and present a 

 remarkable uniformity in structure and functions. We may 

 thus subdivide this primary division into two secondary 

 groups namely, the articulated animals, properly so called, 



Fig. 179.* 



* Nervous system of the aplysia, a gasteropodous mollusc: c, cerebroid 

 ganglions ; g t thoracic ganglions, or sub-cesophageal ; e, nervous collar sur- 

 rounding the gullet; I, labial ganglions ; v, visceral ganglion. 



