28 COMPARATIVE ANATOMY CHAP. 



commissure, which runs over the oesophagus. Each cerebral ganglion 

 gives rise to two powerful nerve trunks which are provided along 

 their whole length with ganglion cells ; there are thus two pairs of 

 nerve trunks running right through the body longitudinally. One pair, 

 the pedal cords, run right and left in the foot ; the other pair, the 

 visceral cords, which lie more dorsally and are more deeply embedded 

 in the body, run through the body cavity. The two visceral nerves 

 are connected posteriorly. 



If we leave the Amphineura and Diotocardia out of the question, 

 the following modified sketch of the Molluscan nervous system holds 

 good. Two cerebral ganglia, two pedal ganglia, two pleural 

 ganglia lying at the sides of the pharynx, two visceral ganglia 

 lying posteriorly in the body cavity. Giving the name connectives 

 to such nerves as unite the ganglia of one side of the body, i.e. dis- 

 similar ganglia, and that of commissures to the nerves that unite the 

 similar ganglia of the two sides of the body, we have the following 

 system: Commissures are found --(1) between the two cerebral 

 ganglia (over the fore-gut) ; (2) between the two pedal ganglia 

 (under the fore-gut) ; (3) between the two visceral ganglia (under 

 the hind-gut). The connectives on each side are: (1) the cerebro- 

 pedal connective ; (2) the cerebropleural connective ; (3) the pleuro- 

 pedal connective ; (4) the pleurovisceral connective. 



There is a secondary ecelom or body cavity lined with endo- 

 thelium, which has at least two divisions. In the anterior division, 

 the genital chamber, the sexual products arise from the endothelium ; 

 this chamber is connected by means of two canals, the genital ducts, 

 with the mantle cavity. In the posterior chamber, or pericardium, 

 lies at least one organ, the heart ; this chamber is connected with the 

 mantle cavity by means of two nephridial duets or vesicles. 



The circulatory system is partly vascular and partly lacunar. 

 The arterial heart lies in the pericardium above the hind-gut. It 

 consists of one ventricle and two lateral auricles. 



II. Review of the Outer Organisation characterising the Chief 



Groups of the Mollusca. 



Having given above a general plan of the morphology of the Mollusca, let us 

 now see how far the various groups of Molluscs agree with this description in their 

 outer organisation. We shall at first only mention in connection with each group 

 those special features which are now considered to be typical or characteristic of 

 that group. In other words, we shall again give a general scheme of the outer* 

 organisation of each class of the Mollusca, in order that these more specialised 

 schemes may be compared with that of the hypothetical primitive Mollusc above 

 described. 



Later sections will deal with the changes which the separate organs undergo, 

 not only in the different classes, but within one and the same class, so far, that is, 

 as these modifications bear on external morphology. 



