APLYSTA. 229 



The retractor penis arises from the muscles of the foot. 

 It consists of numerous bands which join up only at the base 

 of the penis sheath to form the stout hollow pillar of muscle 

 which traverses that organ. (Fig. 19, ret. pn) In the kidney 

 and ctenidium small muscular projections arise from the walls 

 to support the lamellae. (Fig. 21.) 



The musclature of the foot is very characteristic. On the 

 inner surface bordering on the haemocoele, flat longitudinal 

 bands occur and run the whole length of the foot. (Fig. 21, 

 I. ped. m.) Between the bands the haemocoele enters into 

 communication with the lacunar system of the foot. In the 

 spongy tissue of the foot and parapodia the muscle bundles 

 are intricately woven to form a network. The meshes of the 

 network are partly filled with connective tissue of a gelatinous 

 nature, but numerous lacunae occur. The muscles are directly 

 bathed by the blood. 



NEKVOUS SYSTEM. 



(Figs. 16, 17, and 23.) 



The nervous system is of great interest from the compara- 

 tive point of view because it exhibits the intermediate condition 

 between the streptoneurous and euthyneurous types. The 

 Streptoneura possess long visceral cords twisted into a figure 

 of eight, the Euthyneura usually have uncrossed cords, and the 

 majority show a tendency to shortening of the visceral cords 

 and concentration of the chief ganglia around the oesophagus. 

 Aplysia has long, untwisted visceral cords, the homology of 

 whose ganglia is at first sight somewhat obscure and has 

 given rise to considerable confusion in the literature on this 

 Mollusc. 



The nervous system of the typical Gastropod consists of 

 the following ganglia : — Paired cerebral, paired stomato-gastric, 

 paired pedal, paired pleural, paired parietal, and unpaired 



