PTEROPODA AND GASTBOPODA. 539 



estimates at 360,000 ! The head of the Pneumodermon, also a 

 naked Pteropod, is provided with ocular tentacles ; the mouth is 

 covered by a large hood supporting two small simple tentacles, and 

 two large processes beset with numerous pedicellate suckers. There 

 is a small oval foot, with a pointed posterior lobe, beneath the neck, 

 and a rudimental shell protecting the small branchial processes at 

 the bottom of the visceral cavity. 



The cervical aliform expansions of the Clio are muscular, like those 

 of the HycdcBa, subserving locomotion, not respiration, as Cuvier 

 believed. In swimming, the Clio flaps the ends of the wings in 

 contact, first above, then below. The true branchial ciliated and 

 vascular surface is developed in all Pteropods upon the inner surface 

 of the mantle. 



There are two slender and simple tentacula {Jig. 198, 3, k k\ 

 which seem to exercise only the tactile faculty. Two superoeso- 

 phageal ganglia are developed upon the upper part of the nervous 

 collar which incloses the beginning of the alimentary canal ; the two 

 pedial and the two branchial ganglia are closely approximated and 

 connected with the inferior and lateral parts of the nervous collar. 

 From these centres the nerves are distributed to all the viscera and 

 parts of the body. The acoustic vesicles receive their nerves from 

 the suboesophageal ganglion, or the anterior pair, where the mass 

 is subdivided, as in Clio. Hyalaa seems to be blind : most other 

 Pteropods have eyes, and even the Sagitta has a pair of ocelli, 

 forming two prominences on the top of the head, and resting directly 

 on the ganglionic enlargement of the optic nerve. 



The male and female sexual organs are combined in the same in- 

 dividual Clio : the duct of the voluminous ovario-testis communicates 

 with a spherical albuminiparous sac, and is then continued to the 

 base of the intromittent organ, which projects from an orifice on the 

 right side of the head ; it is almost as long as the body, and indicates 

 that impregnation takes place by reciprocal coitus, as in many of the 

 androgynous Gastropods. The sperm-reservoir is a pyriform vesicle 

 with a short peduncle. 



The development of the ova of the Pteropoda has not yet been 

 observed ; but the larvae of some species have been detected ; that of 

 Pneumodermon has the end of the body encircled by two ciliated 

 bands. Vogt afl5rms that two deciduous ciliated cephalic " vela " like 

 those of the larvae of most marine Grastropods precedes the develop- 

 ment of the aliform fins. 



