PTEROPODOUS MOLLUSCS. 443 



opisthobranchiatic, although sometimes affected by the general flexure 

 of the body. The venous system is extremely incomplete. The 

 respiratory organ, which is little more than a ciliated surface, is either 

 situated at the extremity of the body, and unprotected by a mantle, 

 or included in a branchial chamber with an opening in front. The 

 shell when present is symmetrical, glassy, and translucent, consisting 

 of a dorsal and a ventral plate united, with an anterior opening for 

 the head, lateral slits for long filiform processes of the mantle, and 

 terminated behind in one or three points ; in other cases it is conical 

 or spirally-coiled, and closed by a spiral operculum. The sexes are 

 united, and the orifices situated on the right side of the neck. 

 According to Vogt, the embryo Pteropod has deciduous vola like 

 the sea-snails, before the proper locomotive organs are developed." 



The Pteropods seem to be eminently sociable and gregarious, 

 swarming together in great numbers ; they present some analogical 



Figs. 305 and 306. Hj alea gilbosa (Rang.). Figs. 307 and 308. Hyalea longirostris (Lesueur). 



resemblances to the Cephalopodse ; but permanently they represent 

 the larval stage of the sea-snails. De Blainville divides the group 

 into two sections, Thecosomata and Gymnosomata, the first including 

 the HyaleidsB and LimacinidsB ; the second contains one family, the 

 Cliidse. The Hyaleidae have small horny shells, very thin and 

 transparent, globulous, or elongated, open anteriorly, cloven on the 

 sides, and truncate at the posterior extremity. Their globular body 

 is formed of two parts, the one including the head, bearing two very 

 strong tentacles, and two large fins or flappers in the form of wings, 

 springing from each side of the mouth. 



These molluscs are small, and generally of a yellowish-blue or violet 

 colour. They are inhabitants of the deep sea, and rarely seen out of 

 what sailors call " blue water." They plough the waves with great 

 rapidity by the aid of their powerful fins. Certain winds throw 

 them sometimes in great numbers on the shores of the Mediterranean. 



