NUDIBEANCH MOLL USA'S. 



245 



The Nudibranch 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, ySolis (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 



Fig 190. Fig. i91. 



Fig. WO.—/Eolif!,& Nndioranch. 



Pig. 192. 



Fig. 191.— Veliger of Tergipes, «, velum ; s, shell ; d, foot ; h, ot. cysts. — After 

 Schultze. 

 Pig. 192.— Doris bUamellala. 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- T^ig.m.-PAysaheterostropha. Com. 

 ger of Tergipes lacinulata 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 moUusks, Pulmonata, are represented by 

 the pond-snails, Physa (Fig. 193) and Limnceus (Figs. 194, 

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

 «lug suspended by a mucous thread fi'om a twig. 



Tiie common snail. Helix alholalris 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 

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

 den slug {Limax flavus). Fig. 197 represents Helix alho- 

 labris with the shell removed, and the mantle thrown bnck. 



