C(ELUELMi:STUES. 



295 



In structure tho 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 tlie fresh-water GASTROTRICHA {Ichthydium, 

 Chatonotus) and tlie marine ECHINODERIDvE, forms which are little 

 understood. 



PHYLUM YI. CCELHELMINTHES. 



Tho Co'llielmintlios are distingnished from all the forms which 

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

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

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

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



Fig. 2.5fl. — Sfu;tion of Ascarix lvrnhrHimU:s through the pharyngeal bulb; bealde it a 

 bit of the body wall more enlarged, c, cuticle : f/, dorsal line ; h, hypodermis ; j/j, 

 longitudinal muscle; n, nucleus of muscle cell; jj, muscle cell; .s, lateral line; 

 V, A'entral line ; v\ excretory canal. 



outer (parietal) epithelial wall of the cadom and hence are ' epi- 

 thelial muscle cells' (figs. 250, 200). The excretory organs con- 

 nect the body cavity with the outer world and hen(;e are nephridia 

 (earlier called segmental organs, cf. fig. 01)). Internally they begin 

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

 tul.)es expanding just before the enter end to a kind of bladder. 

 The sexual apparatus is simple. The gonads (fig. 200, o) are 



