NUDIBRANCH MOLLUSKS. 245. 
The Nudibranch mollusks, such as the Holis and Doris and 
allied forms, breathe by external gills, arranged in bunches 
on the back, as seen in Fig. 190, Holis (Mon- 
tagua) ptlata (Gould), a common species on 
the coast of New England. In Doris (Fig. 
192), they are confined to a circle of pinnate 
gillson the hinder part of the back. They are 
Fie 190. Fig. 191. Fie. 192. 
Fig. 190.—olis,a Nudibranch. 
Fig. 191. —Veliger of Tergipes, v, velum; s, shell; d, foot; 0, otocysts.—After 
Schultze. 
Fig. 192,—Doris bilamellata. 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 ee 
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- sao, 193 —-Pliyets lateroctrogiie, Com 
ger of Tergipes lacinulata mon vpond-snail.—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 Limneus (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 
at 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, 
