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WORMS. 



By the name "Worms," people commonly indicate a number of 

 different forms whose relations with one another are by no means 

 so close as those of a Holothurian and a Crinoid, or a Mussel and 

 an Octopus. There are not, indeed, any common characters by the 

 possession of which the worm-like animals can at once be distin- 

 guished from other animals. We take the divisions, examples of 

 which are here represented, either by drawings, models, or specimens 

 preserved in spirit separately. 



The groups referred to may be enumerated as follows : — 



Platyhelminthes. 



Nemertinea. 



Nematoidea. 



Chastopoda. 



Platyhelminthes, or Flat-Worms. — These form the lowest and 

 simplest division of the group ; they never have bristles, and are 

 often parasitic in habit. The pai'asitic have been derived from free 

 forms, but parasitism is a habit that leads to great changes in 

 structure ; the Tapeworm, for example, has no mouth. 

 They are divided into — 



I. Turbellaria, free Flat-Worms (1, 2, 36, 37). 

 II. Trematoda or Flukes (29-35). 

 III. Cestoda or Tapeworms (8-28). 



The parasitic Platyhelminthes— the Tapeworms (Cestoda) and the 

 Flukes (Trematoda) — occupy Case I. ; the life-history of the common 

 Tapeworm {Tcmiia solium) is shown by the aid of models and figures. 

 A model of the anterior end of the common Tapeworm shows the 

 four suckers and the crown of hooks ; the unjointed neck is followed 

 by the joints (proffloffids), which increase in size the farther they 

 are from the neck. Several entire specimens of Tcenia follow, 

 showing the size of the whole worm and the form of its joints. 

 The structure of the body is shown in the models of two joints. 

 The growth and development of the Tapeworm is dependent on a 

 migration or a change of the hosts which it inhabits in the various 

 stages of its life ; and although the different kinds of Tapeworm 

 differ from each other somewhat in certain details of their migration 

 and development, their life-history exhibits, on the whole, the same 



