56 MOLLUSCA. 



the sea. The embryos acquire velar lobes and shell, and leave the egg 

 as larva? (fig. 528). While the velum is atrophying, the two fins 

 gradually appear on the first-formed unpaired part of the foot, while 

 the shell (with operculum) is usually cast off. The Hyaleidce how- 

 ever appear to keep the larval shell and develop it further, while the 

 Cymbuliidce replace it by a new shell. The naked Pneumodermidce 

 and Clionidce do not after the loss of the velum and shell grow 

 direct into the sexual animal, but first acquire three rings of cilia and 

 pass into a new larval phase (fig. 529). The Pfceropods always live 

 on the high sea, but may by retracting their velum sink. 



Order 1. THECOSOMATA. 



Pteropoda with a shell. Head but little developed, often not 

 distinct ; tentacles rudimentary. The rudi- 

 mentary foot remains in connection with 

 the fins. 



Fam. Hyaleidae. Shell calcareous or horny, 

 swollen ventrally or pyramidal, symmetrical, with 

 pointed processes. Hyalca tridentata Lam., 

 Cleodora Per. Les., Creseis Rang., Cr. acicula 

 Hang., Mediterranean. 



Fam. Cymbuliidae. With cartilagino-gelatinous 

 shell, boat-shaped or slipper-shaped. Cymbulia 

 Pcronii Cuv., Tiedmannia neapolitana Van Ben. 



Order 2. GYMNOSOMATA. 



Naked Pteropods, head bearing tentacles, 

 tfic.529. Laivnofpneumodermon often with external gills. Fins separated 



(after Gegenbaur). from the f(X)t< 



Fam. Clionidae. Body spindle-shaped, without gills. Clio lorealis Pall., 

 constitutes with Limacina arctica the chief food of Whales. 



Fam. PneumodermonidsB. Body spindle-shaped, with external gills, and two 

 protrusible arms, which are beset with suckers and placed in front of the fins. 

 Pneumodermon violaccum d'Orb. 



Class IV. CEPHALOPODA. * 



With well-marked head, a circle of arms bearing suckers round the 

 mouth and funnel-shaped perforated foot. Dioecious. 



In the form of their body the Cephalopods are most nearly allied 



* Fdrussac et d'Orbigny, " Histoire naturelle generate et particuliere des 

 C^phalopodes acetabuliferes vivants et fossiles." Paris, 1835-45. 

 J. B. Verany, " Mollusques Mediterraneens observes, decrits, figure's et 



