32 



two terminal vessels, the anterior and posterior aortas. The 

 nephridia are coiled tubes which open by one end into the 

 pericardium (coelom), and by the other to the exterior, close 

 to the anus. 



The nervous system of the Archi-Mollusc is rather com- 

 plicated and consists of eleven ganglia and various connecting 

 nerve-cords (see fig. 15. C). There are two cerebral ganglia 

 above and in front of the mouth, and connected by the 

 cerebral nerve commissure. At each side of the stomodaeum 

 is a pleural ganglion, and below it a pair of pedal ganglia. 

 The two pleurals and the two pedals are connected below 

 the stomodaeum by a commissure, and each ganglion is 

 connected also with the cerebral of its own side. From 

 each pleural ganglion a visceral nerve is continued backwards 

 below the alimentary canal to a visceral ganglion from which 

 a nerve is given off to the small ganglion connected with 

 the osphradium at the base of the gill. The two visceral 

 ganglia are joined posteriorly by a visceral loop in the middle 

 of which, below the intestine, a median abdominal ganglion 

 is placed. Finally each pedal ganglion is united to the 

 pleural of its own side and gives off also a posteriorly 

 directed nerve which runs backwards along the lateral part of 

 the foot. A pair of auditory organs (the otocysts) are placed 

 in the front of the foot, and an eye-spot is present at the base 

 of each cephalic tentacle. 



From such an ancestral form as this* all the existing 

 groups of Mollusca have probably been derived. The des- 

 cendants of the Archi-Mollusc must have split into two 

 great divergent series which have resulted in the produc- 

 tion of the Lamellibranchiata on the one hand, and the 

 Cephalophora on the other. 



In the first of these ancestral lines, the primitive Mol- 



* For further details see Eay Lankester's article, "Mollusca," Ency. 

 Brit., 9tli ed., p. 635. 



