\ ERMES 



143 



from twenty to thirty (Fig. 136). The 

 male of this species is only about one 

 millimeter in length and lives as a 

 parasite on the female. In those 

 cases where there is no proboscis, the 

 mouth is surrounded by short tenta- 

 cles or by a tentacular fold, and the 

 whole anterior part of the body can 

 then be withdrawn by means of strong 

 muscles into the large body-cavity; 

 the anterior portion of the body is 

 therefore called the introvert. Those 



Gephyrea hav- 

 ing a proboscis 

 have the anus 

 at the posterior 

 end of the body ; 

 those having an 

 introvert gener- 



FlG. 135. Priapulus. resp, 

 posterior papillae. (After 

 Khiers, from Parker and 

 Haswell's Text-hook.) 



ally have it near 



the anterior end fig. 136. Bomiiiaviridis. Female 

 and on the dorsal wfthpi'obosi ended. 



(After < ireef, from Parker and Has- 



side, the ahmen- weirs Text-book.) 

 tary canal being 



long and twisted within the body-cavity. 

 As there are no dissepiments the body- 

 cavity is a single large chamber. The 

 nephridia are often confined to a single 

 pair, while the blood system resembles in 

 general that of the Chaetopoda. The 

 nervous system consists of a brain dorsal 

 to the oesophagus, a circumcesophageal 

 nerve ring and a ventral nerve cord with- 

 out ganglionic enlargements but with 

 numerous lateral branches. The sexes 

 are separate in the Gephyrea and the 

 development is by means of a trochophore 

 larva, in most instances precisely like the 

 larva of the Polychaeta. In some cases 



