MOLLUSCOUS ANIMALS. 2^ 



In this case, the animal generally retires to some place ot 

 concealment and security, and remains till the shell is re- 

 stored by the deposition of calcareous matter on the exter- 

 ♦nal membrane of the skin, which becomes hard and firm, 

 and finally takes the place of the old shell. 



The Molluscous animals form a large and extensive class, 

 but their structure, residence, and habits, are obscurely and 

 imperfectly known. Among them are the cuttle-fish, oyster, 

 clam, snail, and, in short, nearly all the testaceous animals, 

 or shell-fish, as they are usually called, although they have 

 no resemblance to fishes, and do not all inhabit the water. 

 They are destitute of bones and articulated limbs. Their 

 bodies are generally of a soft texture, and frequently, at first 

 sight, appear to be little else than a simple mucous mass, 

 without parts and almost without organization. In most in- 

 stances they are completely enveloped in a fold or reflection 

 of the skin, which is called their mantle. Sometimes there 

 is only this simple membranous covering ; but more fre- 

 quently there is a hard external shell, which serves as a re- 

 treat into which the animal may withdraw itself, and which 

 it can carry about in all its changes of place. These shells 

 difter exceedingly in shape, colour, and texture, in different 

 species, and among them are found some whose form, polish, 

 and splendid tints place them among the most beautiful ob- 

 jects in nature. 



Questions. — 1. Tn what points do crustaceoiis animals resemble in- 

 sects ? 2. In what do they differ ? 3. What are examples of this 

 class.' 4. What is said of the growth and casting of their shell? 5. 

 What are examples of molluscous animals ? 6. What description of 

 them is given ? 7. What is said of their shells? [Note. The study 

 of those animals in the class mollusca which are characterized by a 

 shell or calcareous covering has obtaitied the distinct scientific name 

 of Conchology. The objects of conchology are separated into three 

 divisions, namely, m2//<i?;«Z?;e5, or shells with many valves ; bivalves^ 

 or sheila with two valves ; univalves, or shells with one valve.] 

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