346 EMBRYOLOGY 



forwards from the posterior pole of the body, thus deter- 

 mining as ventral the side of the body on which this 

 shifting took place. The first cause of this forward migra- 

 tion of the mouth, during which, the opposite edges of the 

 posterior parts of the blastopore successively approached 

 each other until the slit thus produced was at last entirely 

 closed, is to be sought in the significance of the trochal 

 organ as a food-procuring apparatus and the importance 

 of the approach of the mouth toward it. By such a change 

 in the position of the mouth, the relations of the primary 

 axes were disturbed, so that henceforth the chief axis of 

 the body can no longer be referred directly to the primary 

 axis ; for this reason the Bilateria are also designated as 

 Heteraxonia (Hatschek). 



Owing to the resemblance which the Pilidium and many Turbellarian 

 larvte present to the Trochophore, one might also be inclined to derive 

 the Platyhelminthes and Nemertini directly from a Trochophore-like 

 ancestor. The complete ciliation of these forms would then be not 

 primitive, but re-acquired after the transition to the creeping mode of 

 life (therefore as in Coeloplana, comp. p. 157). On the other hand, it 

 must be pointed out that ciliated bands are developed in great variety 

 in pelagic larvas, and we are certainly not in position to prove the 

 homology of all these bands. Hence the resemblance of these larval 

 forms to the true Trochophore of the Annelida is perhaps to be referred 

 merely to analogy in development, which would have its cause in a 

 developmental tendency in this direction inherited from the common 

 ancestor. 



As a result of the development of the most important 

 locomotor organs in the anterior half of the body, it came 

 about that the organs of the animal functions arose in this 

 region. It is this part of the body which, as head, we place 

 in contrast with the posterior, subsequently elongated por- 

 tion of the Trochophore, which is called the trunk, and gives 

 rise particularly to organs of vegetative functions. The 

 fruitfulness of the conception, in the interpretation of the 

 Annelid body, that head and trunk are distinct, has only 

 been equalled by the difficulty of determining the precise 

 boundaries between these two primary regions of the body. 



In the first place, the question arises whether the mouth 



