284 MOLLUSCA. 
freshwater. Some, the sea-slugs or Wudibranchs, lose their 
shells and have an external approximation to plano- 
symmetry. Others are adapted for a pelagic life, they are 
usually transparent, and the shells if present are thin 
and pellucid. The foot is usually reduced. but may form 
a swimming organ. The great majority of the sub-class, 
however, creep on the sea-floor and may be carnivorous 
scavengers, ¢g., whelks, or herbivorous, ¢g., periwinkles. 
The shells of such types as the 
Fig. 200.—A BELEMNITE 7; a 
Rusrorsp. (AfterOwen.) impets and earshells (fa/iotis) are 
; fi not spirally twisted. 
Crass II.—LAMELLIBRANCHIATA. 
The bivalve AZod/usca are usually 
completely enveloped in the paired 
shells. The ctenidia have been 
enormously developed and serve to 
feed the animal. They are mostly 
burrowing types, all aquatic, and 
most are marine. ‘They illustrate 
degrees in degeneration, the oyster 
entirely losing its foot. The scallop 
(Pecten) moves actively through the 
water by snapping its shells together. 
Teredo is a worm-like form with 
very small shells which bores its 
way through wood. Cockles and 
mussels are other common species. 
Cuass III.—CEPHALOPODA. 
min 
In these the molluscan plan 
reaches its highest level. 
Sepia is a very fair type of the 
class. They are all active free- 
g, Eight hooked tentacles (the Swimming forms, with the fore part 
oe oe) fees aes. are of the foot produced into tentacles, 
part containing the shell; 4 the hind part into a siphon, and 
hh: 3; @, ink-sac; é 
Eohon (or funnel. «the organs are plano-symmetric. 
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