36 STUDIES IN BIOLOGY. 



The cerebro-pedal and the cerebro-visceral connectives are fused, or 

 have a common stern, for part of their course in M. latus. 



The cerebral ganglia. These are placed on the dorsal surface of 

 the gullet, rather more than a third of its length from its anterior 

 end, and well apart from each other {eg, Figs. 1 and 17). They are 

 connected by a distinct supra-aesophageal band, the cerebral commis- 

 sure, and give off each two principal branches : (1.) An anterior 

 nerve, which passes along the outer dorsal surface of the gullet, visible 

 from the surface, until it reaches the anterior end of the outer labial 

 palp, when it curves downward (Fig. 1, and a mn, Fig. 19) . It keeps 

 a somewhat direct course downward till it reaches the centre of the 

 thickening of the mantle-edge, then it turns backward and passes 

 along in this thickening till it meets and fuses with a similar nerve 

 from the visceral ganglia (Figs. 1 and 19). This marginal nerve-cir- 

 cuit is the circumpallial or marginal nerve, and the part of it near the 

 cerebral ganglia is the anterior marginal nerve, that near the visceral 

 ganglia the posterior marginal nerve. It forms an additional connec- 

 tion between the cerebral and visceral ganglia, already connected by 

 the cerebro-visceral connective. The course of this nerve resembles 

 that of the circumpallial or marginal artery. (2.) The other main 

 branch from the cerebral ganglia is the stout nerve-cord from each 

 ganglion that passes back along the side of the body to connect with 

 the pedal and visceral ganglia (Fig. 17). Each of these connectives 

 sets out from the outer posterior corner of its ganglion, and passes 

 back nearly along the line of attachment of the inner labial palp of 

 its side and ventral to the main mass of the liver. A little behind, 

 where the palp becomes free, each connective divides into two, then 

 called the cerebro-pedal and the cerebro-visceral connectives, according 

 to their destination. The cerebro-pedal connectives turn in towards 

 the pedal ganglia, which they join (epe, Figs. 1 and 17). The 

 cerebro-visceral connectives run inside the line of attachment of the 

 ascending lamellae of the inner gill till about opposite the foot (c v c, 

 Fig. 2), when they cross to the outer side of this line, and then run 

 along the upper side of the mesosoma, between the lines of attachment 

 of the lamellae of the inner gills. As the mesosoma tapers the cerebro- 

 visceral connectives approach each other, finally joining the anterior 

 ends of the visceral ganglia just behind, and dorsal to the genital 

 papillae on the ventral side of the posterior adductor (Figs. 1 and 2). 

 There are other smaller nerves from the cerebral ganglia, but these I 

 shall describe further on. 



The pedal ganglia. These lie on the dorsal side of the angle 

 formed by the two anterior retractors of the byssus just in front of 



