xxvi THE TROCHOSPHERE 297 



of an intermediate portion or peristomium, having the form 

 of an inverted hemisphere ; and of a lower somewhat conical 

 anal region. Around the projecting rim is a double circlet 

 of large cilia (Pr. or. a) by means of which the larva is 

 propelled through the water. 



Beneath the edge of the ciliated rim is a rounded aperture, 

 the mouth (Mth); this leads by a short, nearly straight 

 gullet (St. dm), into a spacious stomach (Ent\ from the 

 lower side of which proceeds a short slightly curved intestine 

 (Prc. dm), opening at the extremity of the conical inferior 

 region by an anus (An). Between the body-wall and the 

 enteric canal is a space filled with fluid (Bl. cat), but, as we 

 shall see, this does not correspond with the body-cavity of 

 the adult. The body-wall and the enteric canal consist each 

 of a single layer of epithelial cells, all the tissues included in 

 the adult under the head of mesoderm (p. 278) being absent 

 or so poorly developed that they may be neglected for the 

 present. 



Leaving aside all details, it will be seen that the trocho- 

 sphere of Polygordius is comparable in the general features 

 of its organization to a medusa (compare Fig. 55, p. 242), 

 consisting as it does of an outer layer of cells forming the 

 external covering of the body and of an inner layer lining 

 the digestive cavity. There are, however, two important 

 differences : the space between the two layers is occupied by 

 the mesoglcea in the medusa, while in the worm it is a cavity 

 filled with fluid ; and the digestive cavity of the trochosphere 

 has two openings instead of one. 



But in order to compare more accurately the medusa 

 with the trochosphere, it is necessary to fill up, by the help 

 of other types, an important gap in our knowledge of the 

 development of Polygordius the passage from the gastrula 

 to the trochosphere. From what we know of the do^dbp- 



