CLASS V. 

 CONCHOLOaY-MOLLUSCA. 



THIS class is distinguished from the other higher 

 classes by having no bony skeleton which may serve as a 

 solid foundation for supporting the soft parts, as is the 

 case in all the classes already described. Their bodies 

 are soft and slimy, hence their name ; and, for the pro- 

 tective covering of what else would be so defenseless, 

 they are provided with a chalk-like shell, that serves to 

 guard them from contact with the external world. They 

 have cold, white blood; many breathe through organs 

 resembling gills, and in a few, the common snail, for 

 instance, respiration is carried on through sack-like 

 lungs.* 



In most other living creatures, the head, trunk, and 



* The lungs of reptiles consist of a number of bags of membraneous 

 texture, into which the air is conveyed. Most of the mollusca respire 

 by means of gills, which, although they differ in some measure from 

 those of fishes, are formed on the same plan. In some instances they 

 respire air by itself, but in general by the medium of the water alone. 

 In some animals of these classes the gills are situated upon the out- 

 side of their bodies, but commonly within. Zoophytes have no dis- 

 tinct organs for respiration ; yet the air seems, in some way or other, 

 necessary for their existence also, and probably penetrates and acts 

 upon their blood by means entirely unknown. These animals are all 

 cold-blooded. XAT. PHIL. Tr. 



