CCELHELMINTHES. 



295 



In structure the Rotifiers are much like the trochophore type of 

 embryo of annelids and molluscs to be described later. They must hence 

 be regarded as extremely primitive forms, connected at once with the 

 ancestors of these groups, and, as shown by nervous system and excre- 

 tory organs, with the flatworms as well. Most species are cosmopolitan 

 and inhabitants of fresh water. Many species in America. Near the 

 Rotifera may be placed the fresh-water GASTROTRICHA (Ichthydium, 

 Clmtonotus) and the marine ECHINODERID^, forms which are little 

 understood. 



PHYLUM VI. CCELHELMINTHES. 



The Coelhelminthes are distinguished from all the forms which 

 have gone before by the presence of a body cavity, separating the 

 outer body wall from the intestine. This cavity is the coelom, but 

 whether it be homologous in different groups, e.g. nematodes and 

 annelids, is not settled. The body muscles are developed from the 



FIG. 259. Section of Ascari* lumhricoides through the pharyngeal bulb; beside it a 

 bit of the body wall more enlarged, c, cuticle ; d, dorsal line ; 7i, hypodermis ; m, 

 longitudinal muscle; n, nucleus of muscle cell; p, muscle cell; s, lateral line; 

 r, ventral line ; u\ excretory canal. 



outer (parietal) epithelial wall of the coelom and hence are < epi- 

 thelial muscle cells ' (figs. 259, 260). The excretory organs con- 

 nect the body cavity with the outer world and hence are nephridia 

 (earlier called segm en tal organs, cf. fig. 69). Internally they begin 

 with a ciliated funnel, the nephrostome, and continue as long coiled 

 tubes expanding just before the outer end to a kind of bladder. 

 The sexual apparatus is simple. The gonads (fig. 260, o) are 



