MOLLUSCA 



149 



Examination of the young while still in the egg mass will 

 show the beating of the heart. 



Planorbis, another aquatic snail, is distinguished by 

 having all its whorls in the same plane. The forest snail 

 (Helix alboldbris) and the big slug (Limax maximus) are 

 terrestrial. The slugs are characterized by the absence 

 of an external shell and in its stead a plate-like shell buried 

 in the mantle on the back. Slugs and snails breathe by 

 means of lungs, the opening to which is a round hole at 

 the edge of the mantle. Slugs are destructive to garden 

 plants, a common offender being the 

 small dark brown one hiding by day 

 under boards arid logs. 



Cephalopoda: Squids, Cuttlefish, etc. 



The members of this class are so 

 named because a portion of the foot is 

 fused with the head. Around the mouth 

 are several tentacles or arms provided 

 with suckers, serving as powerful organs 

 in seizing prey. In the common squid 

 (Loligo pealii), the lower portion of the 

 foot forms a tube through which the 

 water contained in the mantle cavity 

 may be violently expelled and the ani- 

 mal thus propelled through the water. 

 Into this funnel opens the duct of the. 

 ink sac by which the squid and cuttlefish color the water 

 in order to escape from their enemies. The sense of sight 

 is well developed in the cephalopods, a large eye being 



FIG. 175. The squid. 

 One fourth, natural 

 size. 



