354 MOLLUSC A. 



as organs of smell {(iHpliradiu). They are thickened patches of 

 ciliated epithelia extending into the mantle cavity. Pedal and 

 visceral ganglia are united to the cerebrum Ijy nerve cords, the 

 cerebropedal and cerehrovisceral connectives respectively. Accord- 

 ingly as these connectives are long or short the ganglia are wide 

 apart or united into a nerve mass around the O'soiihagus. 



Pfimitive Mollusoa (Amphineura) have a simpler condition. The 

 cerebral ganglia lie dorsal to the cesopbagus and are united by a cord 

 around the oesophagus (fig, 344). From it are given oflf two pairs of lat- 

 eral nerve tracts, the ventral or pedal cords, and lateral or pleural cords, 

 the latter united by a loop dorsal to the anus. By a concentration of 

 ganglion cells the pedal cords give rise to the pedal ganglia, and similarly 

 tlie pleural cords form three pairs of ganglia, the pleural and the parietal, 

 as well as the visceral already mentioned, of the cerehrovisceral cord (flg. 

 341, A). The pleural ganglia are connected with the pedal by nerve cords; 

 the parietal innervates theosphradium. When farther concentration takes 

 place the pleural may unite with the cerebral, and the parietal with the 

 visceral (fig. 341, B), or both may fuse with the visceral (C). In the latter 

 case the visceral ganglion (in the wider sense) is associated with the pedal 

 by the pleuropedal connective ; while in the other the connective is appa- 

 rently absent because fused with the cerebropedal. Although the otocyst 

 receives its name from the pedal ganglion, the centre of innervation lies in 

 the cerebrum. 



The heart, which lies dorsally, consists of auricles and ven- 

 tricles. The ventricle is always impaired, but there are two auricles 

 where two gills exi,st from which the blood flows to the heart, but 

 with the loss of one gill one auricle may disappear. Distinct arteries 

 and veins occur; capillaries are found only in the Cei^halopoda, 

 while in the lower molluscs, and especially in the Acephala, the 

 smaller arteries open into lacunar spaces which were formerly 

 regarded as the body cavity. A completely closed vascular svstem 

 does not exist even in the Cephalopoda. 



The heart is enclosed in a spacious sac or pericardium, which, 

 with few exceptions, is connected with the nephridia by a ciliated 

 canal, and in many molluscs (Cephalopoda and some Acephala) is 

 also related to the gonads. These facts support the view, already 

 mentioned, that the iiericardium and the lumen of the gonads are 

 the remnants of tlie codom; for here, as in the annelids, the 

 nephridia open by ciliated nephrostomes into the C(elom, and the 

 sexual cells arise eitlier from the ctelomic walls or from sacs cut 

 off from them. Even more important for this view would be 

 confirmation of tlu^ disputed statement that in Pidudina vivipara 

 the coelom (enterocude) arises as diverticula from the archenteron. 



