IV 



ANNELIDA 



151 



mouth and anal openings of the more comphcated animals in general 

 have arisen in evolution — as the isolated and persisting ends of an 

 elongated primitive mouth or protostoma of the type still existing at 

 the present day in such an animal as a sea-anemone. 



The young Polygordius gradually takes on the form of a very charac- 

 teristic tjfpe of larva known as a trochosphere (Fig. 72, A). This is 

 rounded in form, slightly pointed at its anal or posterior end, and pro- 



FiG. 72. 

 Development of Polygordius (A, from Balfour's Embryology ; B and C, 

 after Woltereck). an, Anus ; m, mouth ; n, nephridium ; s, stomach ; 

 s,g, rudiment of supra-oesophageal ganglion ; t, sensory tentacle. 



j acting slightly round the equator. On one side (ventral) is the mouth 

 opening (m) leading into the -[-shaped alimentary canal. The middle 

 part of this is dilated to form a stomach (s) while the oesophagus between 

 the stomach and the mouth is lined by an ingrowth of ectoderm and is 

 consequently stomodaeal in nature. The lining of the alimentary canal 

 is ciliated, while on the outer surface there are powerful cilia round the 

 equator, arranged in a double (pre-oral) band in front of the mouth and 

 a row of smaller (post-oral) cilia behind the mouth, by the movements 



