MICROSCOPICAL STUDIES. 21 



The respiratory region in this species lies on the inner side of 

 the mantle, therefore on what is apparently the ventral aspect of the 

 animal, but which strictly is the posterior. In some species the 

 general surface of the mantle functions, but in this, the chief seat of 

 respiration is a shield-shaped area in the region of the stomach, 

 where the mantle is thrown into curved and transverse folds, bearing 

 cells richly ciliated, whereby the water is kept continuously in 

 motion. 



The central mass of the nervous system is formed by the 

 concentration of three pairs of ganglia around the anterior end of the 

 oesophagus ; that part lying above, representing the supra-cesophageal 

 ganglia ; that beneath, of two pairs, named respectively the 

 visceral and the pedal ganglia. Nerves going to the mantle and 

 to the alimentary organs can readily be traced proceeding from the 

 hinder part of the nerve-mass, but in size, these are far surpassed by 

 two enormous nerves (/. n.) given off, one on either side, by the pedal 

 ganglia, for the nerve supply of the swimming fins. Each on entry, 

 throws off a smaller branch, and then proceeds to give off with 

 remarkable regularity, some 20 pairs of lateral nerves at short 

 intervals. Each pair consists of a right and a left nerve originating 

 from the same point. The same arrangement is repeated by the 

 smaller branch. 



For so small an animal, the reproductive organs are extremely 

 complex and are complicated by the creature, like all Pteropods, being 

 hermaphrodite. Ova and spermatozoa are produced in the same 

 gland, the ovo-testis or hermaphrodite gland. This lies, as a compact 

 elongated mass, in the hinder (dorsal) end of the body, parallel with 

 the liver. It is connected by a fine efferent duct with the sexual 

 orifice which opens on the right side just dorsal to the base of the 

 right fin. Connected with the lower end is a side pouch — the uterine 

 caecum. Another pouch-like organ of equally great size, opens close 

 by the genital aperture, and just at the base of the right rudimentary 

 tentacle (p. g.) This is the invaginated (indrawn) penis or penial 

 gland, the external male sexual organ, highly specialised as in the 

 gastropod molluscs, and here as in other Pteropods, of very large size, 

 The spermatozoa pass from the common sexual orifice to the penis by 

 a short ciliated external gutter or furrow (c. /.) Self fertilization is 

 obviated by the male and female elements maturing at .different 

 periods. When copulation with another individual takes place, the 

 great penis is evaginated and inserted into the uterine caecum, the 

 spermatozoa pass in, and thence are conveyed to a small vesicle, the 

 receptaculum seminis, there to await the arrival of mature ova from the 

 hermaphrodite gland. When this occurs, the sperm escapes from its 



