CLASSIFICATION OF GASTEROPODS. 329 



Appendix to the Gasteropoda, probably to be included among 

 them ; — 



Pteropoda. Winged Snails, Sea-butterflies. 



Free-swimming pelagic molluscs, with secondarily acquired bilateral 

 symmetry. They swim by two large side lobes of the foot ("epipodia"), 

 the median region being much reduced. The visceral dome is long, 

 rarely in a spiral, sometimes covered with a light shell, sometimes naked. 

 There are no gills. Anal, nephridial, and genital apertures lie ante- 

 riorly, and usually on the right. The animals are hermaphrodite. They 

 are carnivorous in habit, often swim actively in shoals, and occur in 

 all seas. They afford food for whales, etc. , and the shells of some are 

 abundant in the ooze. 



Examples — 



Thecosomata, with mantle fold and shell ; — Hyalea, Cymbulia. 

 Gymnosomata, without mantle fold or shell in the adult ; — Clio, 

 Pneumodermon. 



Class SCAPHOPODA. 



Very different from all Gasteropods are the Scaphopoda, of which 

 Dentalium (Elephant's tooth shell) is the commonest genus. Their 

 position is uncertain. Some place them nearer bivalves, others nearer 

 Cephalopods. They live at considerable depths off the coasts of many 

 countries. The tubular shell is shaped like an elephant's tusk, and is 

 open at both ends. The anterior end is directed downwards in the sand. 

 The concave side is dorsal, the convex ventral. With the form of the 

 shell the state of the mantle corresponds, for its two folds, originally 

 separate as in bivalves, fuse into an almost complete tube. The 

 shell also is at first an incomplete tube. The small cylindrical head 

 bears at its extremity a mouth surrounded by numerous "tentacles," 

 while at the base of the head there is a double cluster of ciliated con- 

 tractile processes possibly representing gills. The foot is long, with 

 three small' terminal lobes. It is used in slow creeping. There are 

 cerebral, pedal, and pleural ganglia, near one another in the head, and 

 the visceral loop is long. Sense-organs are represented by otocysts 

 beside the pedal ganglia. There is an odontophore with a simple 

 radula. The food consists of minute animals. There is no heart, but 

 colourless blood circulates in the spaces of the body. There are two 

 nephridial apertures, one on each side of the anus ; the nephridial 

 chamber is perforated by the intestine. The sexes are separate ; the 

 reproductive organ is simple and dorsal in position ; the elements pass 

 out by a duct which opens into one of the nephridial ducts. The gastrula 

 is succeeded by a free-swimming st^e, in which there is a, hint of " 

 velum and a rudimentary shell-gland. 



Examples. — Dentalium, Entalium. About forty widely distributed 

 species are known. Dentalium entale occurs off British coasts. 

 The genus occurs as a fossil from Carboniferous (or perhaps 

 earlier) strata onwards. 



