60 NATURAL HISTORY. 



limbs are easily recognized, because their organization 

 approaches more nearly that of the mammalia. In the 

 most of the mollusca such an arrangement of organs is 

 not apparent ; and it requires an experienced eye to dis- 

 cover and decide -what is the head, trunk, or feet. Their 

 bodies are generally covered by a fold or reflection of the 

 skin, -which envelopes them completely, and is called 

 their mantle; sometimes this covering is membranaceous, 

 or partly fleshy, but in most cases is composed of a stony 

 matter resembling carbonate of lime. This envelope 

 serves to protect the soft parts of their bodies, and in 

 many of the Crustacea exhibits most beautiful varieties 

 of form and color. This order is divided into two class- 

 es, partly from their organs of motion, and partly from 

 the form and structure of their shells. The following 

 table will give an idea of their classification. 



FIRST ORDER : . . CEPHALOPODA. The fec-t or teutacula go out from 

 the head ; somewhat resemble arms, and are fur- 

 nished with cups or suckers ; the Cuttlefish be- 

 longs to this order. 



SECOND ORDER : GASTEROPODA. Includes those mollusca that 

 have the soft portions of the body inclosed in a 

 shell, consisting of many convolutions, and the 

 feet, or instruments of motion, are placed in the 

 form of a fleshy mass under the belly. The 

 Snails belong to this order. 



THIRD ORDER : . ACEPHALJ The mollusca belonging to this class 



have no distiact heads ; neither can trunk or 

 limbs be distinguished from the general mass. 

 This order includes all the bivalves, such a? oys- 

 ters, clams, etc. 



