SOME PROBLEMS OF ANNELID MORPHOLOGY. 7/ 



tlie same 07itogcnetic origin^ in the cleavage-process, as the 

 viesoblast in other annelids. In other words, the meso- 

 blast is differentiated during the cleavage in essentially 

 the same way as in other forms, but is not removed from 

 the surface until a very late period. It forms, in fact, 

 a part of the outer layer of the larva, which has accord- 

 ingly a deceptive appearance of being two-layered. In 

 reality the third layer is already present ; and it con- 

 stitutes here, as elsewhere, the germ of the trunk. 



It appears, therefore, that in all cases the trochophore 

 contains a rudiment of the trunk, the presence of which 

 means in my opinion, that the larva once possessed a 

 fully developed trunk, which is now temporarily reduced 

 in favor of the head. A somewhat analogous case is 

 that of the Nauplius larva of the Crustacea. This larva 

 has but three pairs of functional appendages, which be- 

 come highly organized and of great functional impor- 

 tance while the remaining appendages are represented 

 by mere rudiments, or, it may be, by mere groups of 

 cells near the posterior extremity of the larva. Now, 

 it is nearly certain, in the opinion of the best authori- 

 ties, that the Nauplius is a secondary form ; that the 

 posterior appendages formerly developed in uniform 

 succession to the three anterior pairs ; and that their 

 temporary suppression in the Nauplius has been sec- 

 ondarily brought about. This case, though not entirely 

 parallel to that of the trochophore, will serve to illus- 

 trate the general character of the change which I believe 

 the latter form has undergone. 



Let us finally pass in brief review the principal points 

 we have considered. Of the three main problems sug- 

 gested by the development of annelids, only one can 

 at present be brought under a satisfactory working 



