82 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



which work like paddles. The body is not contractile, 

 as in the jellyfishes. They are considered the highest of 

 coelenterates, having a complex nutritive apparatus and 

 a definite nervous system. There is no trace of a polyp 

 stage in their development, and they do not form colo- 

 nies. They are found in all regions of the ocean, from 

 the arctics to the tropics. 



Branch IV. PLATYHELMINTHES 



The group formerly called Vermes or worms was 

 composed of animals so very different in form and 

 structure that it has now been subdivided into several 

 branches, viz. : Platyhelminthes, or flat worms ; Ne- 

 mathelminthes, or round worms ; Trochelminthes , or 

 rotifers ; Molluscoida, including the Polyzoa and Bra- 

 chiopoda ; and Annulata, or segmented worms. All 

 these forms agree in being distinctly bilaterally sym- 

 metrical animals, as contrasted with the apparently 

 radial arrangement of parts seen in the C&lenterata 

 and EC kino derma ta, and in having the three body layers 

 ectoderm, mesoderm, and endoderm well developed, 

 the mesoderm or middle layer being relatively of more 

 importance than in any preceding group. 



The Platyhelminthes, or flat worms, include some free 

 forms, as Planaria, which is common in fresh water, 

 and the tapeworms and flukes among the parasites. 

 As a group, they are soft-bodied, flattened animals, 

 without skeletal parts of any kind. There is no distinct 

 body cavity nor blood-vascular system nor anal opening. 

 The digestive system may be entirely absent, as in the 

 tapeworm, or it may be much branched and highly 

 complicated in structure, as in the planarians. 



The tapeworm ( Tcenia) consists of the so-called head 

 and the body segments, which are really reproductive 



