264 



COMPARATIVE ZOOLOGY. 



It has also close relations with the other subkingdoms of 

 the bilaterally symmetrical animals. Through the Poly- 

 zoa and Brachiopoda, it approaches the Mollusca ; through 

 the Annelides, the Arthropoda ; and through other forms, 

 the Tunicata, and so the Vertebrata. The subkingdom 

 thus stands in the centre of several subkingdoms, with 

 affinities towards all. Nor are indications of connection 

 with Coelenterata and Echinodermata wanting. 



The Vermes are bilaterally symmetrical animals, with one 

 or many segments, no jointed legs. They usually have a soft 

 skin, and peculiar excretory organs — the segmental organs. 



Many of the "Worms are parasitic, and most of the en- 

 doparasites belong to this group. 



There are numerous classes, of which only the most im- 

 .portant are mentioned. 



Class I.- 



Platyhelminthes. 



The 



Flat - worms 

 include some free 

 forms, as the Plana- 

 ria, common in fresh 

 water, and the Tape- 

 worms and Flukes 

 among the parasites. 

 The Tape - worm 

 consists of the so- 

 called head — the 

 proper worm — and 

 the body segments. 



Fig. 216 — Tape-worm {Tcsnia ttoliuw): a, head; 6, c, 

 d, segments of the body. 



Pio. 21T.— Plauariaa 

 worm. 



