22 GENERAL ZOOLOGY 



tive system; a very small coelom present. A well-developed 

 system of excretory tubes present. Flatworms : planarians, 

 flukes, tapeworms. 5000 species. 



6. Nemertina. Flattened worms which resemble the 

 Platyhelmia in general appearance, but are more special- 

 ized. An anus is present at the posterior end of the body. 

 The nemerteans. 280 species. 



7. Nematoidea. (Nematos, a thread). Cylindrical or 

 thread-like, bilaterally symmetrical, non-met americ, triplo- 

 blastic worms. A mouth and anus are present and the 

 body is of the " tube-within-a-tube type." The body cav- 

 ity is, however, unlined and hence not a true ccelom. 

 Round- worms: Trichina, hook-worm, vinegar-" eels," etc. 

 1500 species. 



8. Rotifera (rota, a wheel; /ero, I carry). Microscopic, 

 bilaterally symmetrical, metameric, triploblastic animals; 

 with a coelom; two ciliated lobes, which often look like 

 rotating wheels, at the anterior end. The digestive system 

 is tubular, with a mouth, anus, and several distinct regions. 

 There are chewing teeth in the stomach. A pair of water 

 canals are present which serve for excretion. The wheel- 

 animalcules. 500 species. 



9. Brachiopoda (brachion, the arm; pous, a foot). 

 Marine, bilaterally symmetrical, triploblastic animals; 

 which possess a bivalve shell (valves dorsal and ventral) 

 and are usually attached by a stalk. A peculiar horseshoe- 

 shaped structure in front of the mouth gives the name to 

 this phylum. The body is very small in proportion to the 

 size of the shell. The lamp shells, which are mostly extinct, 

 are examples. 500 living species. 



10. Bryozoa (6rwm, moss; zoon, animals) . Small, usually 

 colonial animals, which are mostly marine. The colony 

 usually has a branching plant-like form. At the tips of the 

 branches are the individual animals which have a super- 

 ficial radial symmetry. A number of tentacles surround 

 the mouth and a U-shaped digestive tube leads to the 

 anus. 1200 species. 



