162 A COURSE ON ZOOLOGY. 



It must not bo thought that worms are the only 

 species that live as parasites. .Parasitic animals arc 

 found among insects, such as lice, fleas, etc. ; among 

 spiders, as the ascaris or itch parasite ; and among crus- 

 taceans. Parasitism is only the condition of a certain 

 number of creatures incapable of supplying their own 

 wants. We will return to the subject when considering 

 animals injurious to man. 



We will now resume the generalities on the organiza- 

 tion of annelida in the following table, which indicates 

 the different groups : 



(ganglionic ; well developed. Annelides. 



f non-parasitic : mouth furnished 

 with vibratile cilia. Rotatoria. 



rudimentary j f ribbon-like. Cestoidea. 



parasitic ; no I cylindrical. Nematoidea. 

 I cilia ; 3 forms flattened. Trematoidea. 



CHAPTER XVIII. 

 General Ideas concerning Mollusks. 



THE mollusks have no internal skeleton like the ver- 

 tebrates, nor an external skeleton formed by a harden- 

 ing of the skin, as in a large number of articulates 

 and arthropods; this is indicated by the name mollusk, 

 which means soft animals. Most of them are, however, 

 provided with a shell, which serves for protection, and 

 represents a sort of skeleton. 



The shell is either univalve or bivalve as it is composed 

 of one or two pieces. Whatever its form may be, it is 

 always a product of secretion of the skin. 



