HABITS, ETC., OP CEREBRATULUS LACTEUS. 187 



when taken together furnish strong evidence that the fibres 

 are muscle. Whether the strand also contains nerve-fibres 

 must remain an open question ; none can be seen, and 

 pilidiuras kept for days in water impregnated with methylene 

 blue showed no trace of coloration. 



Anteriorly the cilia rows, in passing from the lappets to the 

 oral surface, invaginate in a large loop (fig. 82), which pro- 

 jects into the central cavity in the same manner as the apical 

 plate. 



The two loops are composed of the same sort of cells as 

 the rest of the cilia rows, and appear undifferentiated. They 

 appear about the sixth day, and increase in size up to the 

 eighth or ninth day, after which they remain unchanged 

 as long as I have succeeded in keeping any larvge — about six 

 weeks. 



I believe these to be the first pair of invaginations which 

 are to form the adult Nemertean, but, of course, have been 

 unable to prove the matter. 



The Intestinal Canal. — In the mature pilidium this is 

 composed of two parts, each consisting of a single layer, and 

 well differentiated. At first the intestine is composed through- 

 out of large cylindrical cells, but as soon as the inner end 

 begins to turn down posteriorly the constituent cells begin 

 to differentiate. Those on the anterior and lateral walls of the 

 oesophagus fiatten out, increase in superficial area, and become 

 transparent in a very similar manner to the aboral ectoderm. 



But the process is not carried so far, and the cells never 

 lose their vitality. The flattening is not uniform, so that the 

 cells grow thinner from the stomach toward the mouth, where 

 they pass insensibly into the oral ectoderm. 



The posterior wall of the oesophagus is thicker, its cells are 

 smaller in area, more cylindrical in form, and do not become 

 as transparent. They are also depressed along the mid-line 

 into a shallow groove (fig. 82), which forms a channel for the 

 food particles. 



The stomach wall is thicker than that of the oesophagus, 

 its component cells are larger and less transparent, and are 



