NUDIBBANCH MOLL USES. 



265 



The Nudibraucli mollusks, such as the Eolis and Dorin and 

 allied fiirins, breathe by external gills, arranged in bunches 

 <>n the back, as seen in Fig. 195, jl<Jolis (Mon- 

 tagua) pilata (Gould), a common species on 

 the coast of New England. In Doris (Fig. 

 196), they are confined to a circle of pinnate 

 gills on the hinder part of the back. They are 



Fig. 195. 



Fio. 196. Fig. 197. 



velum ; ,«, shell ; d, foot ; b^ otocysts. — After 



Fig. 198. — Physa ^^terosiropha. Com- 

 mon pond-snail. — After Morse. 



Fig. 195.— .Fofis.aNndibraiich. 

 Fig. 196. — Veliger of Tergipes 

 Schultze. 

 Fig. 197.— Doris. New England coast. 



shelless, and not uncommon just below low-water mark, 

 laying their eggs in jelly-like masses coiled up on stones and 

 the surface of sea-weeds. Though the adults are shelless, 

 the embryos at first have a shell 

 (Fig. 196, s), indicating that 

 the Nudibranchs have descend- 

 ed from shelled G-astropods. 

 Fig. 196 represents the veli- 

 ger of Tergipes lacinulata 

 Schultze, allied to Doris, with its large ciliated velum, and 

 protected by a deciduous shell, which finally disappears with 

 the velum. 



The air-breathing mollusks, Pulmonata, are represented by 

 the pond-snails, Physa (Fig. 198) and Limnwns (Figs. 199, 

 200), and the land-snails and slugs. Fig. 205 represents a 

 slug suspended by a mucous thread from a twig. 



The common snail. Helix alholahris Say, is a tjrpe of the 

 air-breathing mollusks. Fig. 301 represents this snail of 

 natural size, in its shell. The opening to the lung is seen 

 at a, and at B is represented the heart and lung of the gaiN 

 den slug (Limax flavus). Fig. 202 rejjresents Helix alio- 

 Inbris with the shell removed, and the mantle thrown back, 



