38 • STUDIES IN BIOLOGY. 



Smaller nerves. Each cerebral ganglion gives off between the 

 anterior marginal nerve and the cerebro-visceral connective two small 

 nerves (Fig. 17), the anterior of which goes to the outer labial palp 

 of its side, and the posterior one goes almost directly outward 

 towards the inner labial palp, giving off a small branch on the anterior 

 side. There is also a pair of small nerves given off between the an- 

 terior marginal nerves. The anterior marginal nerves give off a small 

 branch into the mantle. 



Each pedal ganglion gives off four nerves behind the cerebro-pedal 

 connectives. The most anterior is a small nerve that becomes lost in 

 the connective tissue round the retractors of the foot (Figs. 17 and 20). 

 From the outer and hinder corner of each ganglion two nerves are 

 given off ; the anterior is very large, and enters the foot, being the 

 pedal nerve ; the posterior of the two is smaller, and supplies the base 

 of the byssus and the anterior part of the mesosoma, being the byssal 

 nerve (b n, Figs. 17 and 20). From the posterior side of the ganglia 

 a pair of small nerves arises, the nerves being close together. These 

 supply the middle and posterior retractors (Figs. 17 and 20). 



The visceral ganglia give off on the anterior side between the 

 cerebro-visceral connectives a pair of fine nerves that run forward. 

 These I have traced into the connective tissue round the posterior re- 

 tractors of the byssus. Rising from the same part is a pair of delicate 

 nerves that supply the dorsal edges of the mantle and have a very 

 curious course. These nerves (the dorsal marginal nerves) rise from 

 the anterior dorsal surface of the visceral ganglia, and, taking an 

 oblique course upwards in front of the posterior adductor, pass 

 between the posterior retractors {dmn, fig. 19). They continue this 

 slanting course upwards till they arrive at the mantle-edges above the 

 pericardium, where they form marginal nerves. I cannot find a 

 similar pair of nerves mentioned in any of the works to which I have 

 access, and it is peculiar that nerves to supply an outlying part like 

 the mantle should take their course through so great a length of the 

 body proper. Each visceral ganglion also gives off on its upper side 

 a nerve to the posterior adductor, and these nerves vary, being some- 

 times single, sometimes forked. There is a small nerve from the 

 outer side of the visceral ganglia to the mantle, following the line of 

 attachment of the ascending lamellae of the outer gill. 



There is a marginal festoon of nervelets between the first and 

 second posterior marginal nerves. 



The cerebro-visceral connectives give off a small nerve a little 

 behind the middle retractors (Fig. 17). 



The nervous system of M. latus (Fig. 17) resembles that of M. 



