INCIStFEA (sCISSURELLA) LYTTELTONENSIS. 5 



multispira,!, as in other Scissurellidas. It must be regarded 

 as vestigial since, as is the case in Pleurotomaria, it cannot 

 be of any use in closing the aperture of the shell. There are 

 two columellar or shell-muscles (fig-. 2) symmetrically disposed 

 right and left of the middle of the body, the right muscle 

 being slightly larger and extending rather further back than 

 the left. 



As it is almost impossible to make dissections of an animal 

 scarcely exceeding 1 mm. in length, the following account of 

 the anatomy of Incisura is mainly founded on reconstructions 

 from sections, but I succeeded in making some satisfactory 

 whole preparations of the ctenidia, and have checked the 

 results of my reconstructions as far as possible by the study 

 of whole specimens cleared in various ways. Fig. 2 is a 

 camera drawing of a specimen stained in picro-carmine and 

 mounted in oil of cloves ; it shows as much of the general 

 anatomy as can be made out by this method. Figs. 3, 4, and 

 5 are reconstructions from sections showing respectively the 

 anatomical relations of the alimentary tract, the kidneys and 

 pericardium, and the nervous system. Figs. 6 to 12 are 

 camera drawings of some of the sections from which the re- 

 constructions were made. 



Organs of the pallial complex. — Incisura is typically zygo- 

 branchiate, and the position and general characters of the 

 ctenidia, hypobranchial glands, left kidney, and pericardium 

 have been correctly described by Pelseneer. 



The ctenidia. — Both right and left ctenidia take their 

 origin from the roof of the mantle-cavity, close to the anterior 

 end of the columellar muscle of their respective sides of the 

 body. The left ctenidium lies almost transversely across the 

 neck of the animal, its anterior extremity reaching nearly as far 

 as the base of the right tentacle (fig. 2), and it is closely com- 

 pressed between the body-wall and the roof of the mantle. 

 The right ctenidium, on the other hand, lies for the most 

 part in front of the right columellar muscle, and the bulk of 

 it hangs vertically downwards in the space enclosed between 

 the mantle and the outer side of the foot (fig. 7). Pelseneer 



