NUDIBRANCH MOLL UtiKS. 



215 



The Xudibranch mollusks, such as the Eolis and Doris and 

 allied forms, breathe by external gills, arranged in bunches 

 on the back, as seen in Fig. 190, sEolis (Mon- 

 tagua) pilata (Gould), a common species on 

 the coast of New England. In Doris (Fig. 

 192), they are confined to a circle of pinnate 

 gills on the hinder part of the back. They are 



Com- 



FIG. 190. FIG. 191. FIG. 192. 



Fig. 190. <Ww,a Nudihranch. 



Fig. 191. Veliger of Tergipes, v, velum ; s, shell ; d, foot ; b, otocysts. After 

 Schultze. . 



Fig. 192. Doris bilameUuta. 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. 191, s), indicating that 

 the Nudibranchs have descend- 

 ed from shelled Gastropods. 

 Fig. 191 represents the veli- Fi? _ m .-ph ygai 



ger of Tergipes ladnulata mon pond-enail. After Morse. 



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. 193) and Linumus (common in 

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

 slug suspended by a mucous thread from a twig. 



The common snail, Helix albolabris Say, is a type of the 

 air-breathing mollusks. Fig. 196 represents this snail of 

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

 ut a, and at B are represented the heart and. lung of the gar- 

 den slug (Limax flavus). Fig. 197 represents Helix albo- 

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



