MOLLUSC A 313 



cavity. Pedal and visceral ganglia are united to the cerebrum by nerve 

 cords, the cerebropedal and cerebramsceral connectives respectively. Ac- 

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

 apart or united into a nerve mass around the oesophagus. 



Primitive Mollusca (Amphineura) have a simpler condition. The cerebral 

 ganglia are connected by a ring around the oesophagus (rig. 315, B). From it 

 are given off two pairs of longitudinal 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 in the higher molluscs the pedal cords give rise 

 to the pedal ganglia, and similarly the pleural cords form three pairs of ganglia, 

 the plcural and the parietal, as well as the visceral already mentioned, of the cere- 

 brovisceral cord (fig. 312, A}. The pleural ganglia are connected with the 

 pedal by nerve cords; the parietal innervates the osphradium. When farther 

 concentration takes place the pleural may unite with the cerebral, and the par- 

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

 case (pulmonates, cephalopods) the visceral ganglion (in the wider sense) is 

 associated with the pedal by the pleuropedal connective; while in the other 

 (lamellibranchs, scaphopods) the connective is fused with the cerebropedal. 

 Although the statocyst receives its nerve from the pedal ganglion, the centre of 

 innervation lies in the cerebrum. In the Nuculidae the statocysts retain their 

 connection with the parent ectoderm by means of a canal, which though closed, 

 remains in part in the Cephalopods. Besides accessory eyes in various places, 

 there are cephalic eyes, in general structure like those of the annelids. They are 

 pits in the skin, the bottom differentiated to a retina. Usually they close to a 

 vesicle, but only in the cephalopods do they reach a high development (fig. 



349)- 



The heart, which lies dorsally, is arterial and consists of auriclap and 

 ventricles. The ventricle is always unpaired; there are two auricles where 

 two gills exist 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 Cephalopoda, while in the lower molluscs 

 (especially Acephala), the smaller arteries open into lacunar spaces which 

 were formerly regarded as the body cavity. A completely closed vascular 

 system 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 

 (nephrostome), and in many molluscs (Cephalopoda, Solenogastres) 

 is also related to the gonads. These facts support the view that the 

 pericardium and the lumen of the gonads are the remnants of the ccelom; 

 for here, as in the- annelids, the nephridia open by ciliated nephrostomes 

 into the ccelom, and the sexual cells arise either from the ccelomic walls 

 or from sacs cut off from them. 



Nephridia and sexual organs are primitively paired, but frequently 

 are single by the degeneration of those of one side. The animals are 

 either hermaphroditic or dioecious, but the gonads are always very large. 



