240 



HOW DO WE CLASSIFY ANIMALS? 



Self-Testing Exercise 



Arthropods are animals which have (1) (2) and legs 



and have a hard (3) (4) made of either (5), 



(6), or both. There are four common (7) ; crus- 

 taceans, which live mostly in (8) and breathe by means of 



(9) ; insects, a (10) group, which has the (11) 



divided into (12) parts, and has three (13) of jointed 



(14) ; the myriapods which have ......... (15) bodies with 



(16) pairs of (17) ; and the arachnida, spiders, with 



. . (18) pairs of legs and no ........ (19), and (20) 



pairs of simple eyes. The spiders spin (21) from silk which they 



(22) by means of glands. 



PROBLEM IV. WHAT ARE THE CHARACTERISTICS OF THE 



MOLLUSKS? 



Most mollusks have shells composed mostly of lime, either 

 bivalve (two-valved), as the oyster, clam, mussel, and scallop, 

 or univalve (with one valve), as the snail. Usually the univalve 

 shell is spiral in form. Inside the shell, which is formed by a 

 F . . delicate structure called 



the mantle, is found the 

 soft, unsegmented body, 

 from whence it gets the 

 name mollusk (Latin mol- 

 lis, soft). Other mollusks, 

 for example the garden 

 slug, have no shell what- 

 ever, and one highly 

 specialized form, the squid, 

 has an internal shell. 



Pelecypods. Between 

 the mantle and the body 

 of the mollusk is a space, 

 the mantle cavity, in which 

 hang the platelike gills. 

 By means of cilia on the 

 mantle and gills, a con- 



A fresh-water mussel (clam) half buried in the mud. £ ..J 



Explain how it moves and how it gets its food. Slant Current 01 Water IS 



'ipKons- 



ouL 



^vfcrter 



