iNCisuuA (scissurella) lytteltonensis. 27 



The visceral loop bears three distinct accumulations of 

 ganglion cells^ forming as many ganglia. The i-ight ganglion 

 lies close below the gonaduct and gives off a slender nerve to 

 that organ. The pedal ganglia, as may be seen in fig. 5, are 

 very much concentrated. Anteriorly they are rather flat, but 

 in about the middle of their length they increase considei'ably 

 in thickness, this increase being due to the addition of a 

 considerable ventral thickening to each ganglion. In this 

 region, in fact, each pedal ganglion consists of a dorsal and a 

 ventral moiety, as is the case in all Rhipidoglossa (fig, 0). 

 Here also the whole of the pedal ganglia lies in the haemocoele, 

 as is the case with the more elongated pedal cords of Fissurella. 

 But in Incisura the dorsal moieties of the pedal ganglia have 

 very little posterior extension. The ventral moieties, on the 

 other hand, extend back behind the dorsal inoieties, and, 

 narrowing in diameter, plunge into the muscular substance of 

 the foot (tig. 10). There they are continued backwards for a 

 short distance, giving off nerves from their outer edges, and 

 diminishing rapidly in diameter, partly because of fibrils 

 given off to the different nerves, but also largely because of 

 the thinning out and eventual disappearance of their coating 

 of nerve ganglion cells. Posteriorly the cords become simple 

 nerves, and end some distance in front of the posterior end of 

 the foot. Pelseneer states of Scissurella costata and 

 Incisura lytteltonensis: "Dans les deux especes, les 

 cordons pedieux sont dans la masse nmsculaire du pied, et 

 s'eLendent jusqu'a la partie posterieure." This is certainly 

 not the case in Incisura; the left pedal cord, or rather nerve, 

 dies out at a distance of 125 /n from the posterior end of 

 the foot in two specimens in which I calculated its extent, and 

 temembering that the animal is only 1 mm. long this is a 

 considerable distance. In short, one can hardly speak of 

 pedal cords. The pedal centres, particularly the dorsal 

 portions of them, have become concentrated into two clearly 

 defined pedal ganglia, and it is only the ventral portions that 

 are continued backwards to represent in some measure the 

 elongated pedal centres of other Rhipidoglossa. In addition 



