PTEROPODA. 



239 



A common form, occurring at the surface in harbors 

 north of Cape Cod, as well as many miles off shore, is Spiri- 

 alis Gouldii Stimpson, the shell of which 

 resembles a conical Helix. The largest 

 form on the eastern coast of North 

 America, extending from New York to the 

 polar seas, is the beautiful Glione papillon- 

 acea of Pallas, which has a head and lin- 

 gual ribbon. It is rare on the coast of 

 New England, but abundant from Labra- 

 dor northward. We have observed it 



rising 



and falling in the water between 



Fig. 178. Styliria 

 vilrea. After Verrill. 



the floe-ice on the coast of Labrador. It 

 is an inch long, the body fleshy, with no 

 shell, the wings being rather small. 



The larvae of the Pteropods pass through 

 a trochosphere stage, being, as in Cavolina, 

 spherical, with a ciliated crown. It after- 

 wards assumes a veliger form. Fig. J 79 represents a worm- 

 like, segmented, Pteropod larva, the adult of which is 

 unknown. In other genera the larvae are annulated, resem- 

 bling the larvae of Annelides. 



The Pteropods are, in some degree, a generalized type. 

 They have a wide geographical distribution and 

 a high antiquity ; forms like Cavolina, viz. : 

 Tlieca, Comdaria, Teniaeulites, Cornulites, 

 etc., dating back to the palaeozoic formation ; 

 Theca-like forms (Puyiunculus and Hyolitlies) 

 occurring in the primordial rocks. 



Order 3. Gastropoda. This great assemblage 

 of mollusks is represented by the sea-slugs, 

 limpets, whelks (Figs. 180-183), snails, and 

 Fig. i79.-ptero- slugs. The head is quite distinct, bearing one, 



pod larva. -. . . . . , , , ., 



and sometimes, as in the land-snails, two pairs 

 of tentacles, with eyes either at the bases, or at the ends of 

 the tentacles, or, as in Trivia californica (Fig. 184), they 

 are situated on projections near the base of the tentacles. 

 All the Gastropods move or glide over the surface by the 

 broad creeping-disk, a modification of the foot of the clam, 



