728 PROCEEDINGS OF THE ACADEMY OF [Oct., 



striking, viz., in the origin of the bilateral cleavages. In the cross 

 and in the products of 2d the transition from the spiral type of 

 cleavage to the more specialized bilateral type occurs in precisely 

 the same cells and in precisely the same directions as in the Poly- 

 chseta. Moreover, the second bilateral divisions of the cells of the 

 posterior arms of the cross continue this resemblance. All these 

 characters, if such they may be called, when viewed as a whole, 

 point in no uncertain way to a descent from the annelid stem, and at 

 a point not far from that at which the Polychaeta arose. 



If we consider, as I think at present we must, that the trochophore 

 is a larval form common at least to the Annelida, we cannot regard the 

 development of Dinophilus as primitive. This view is upheld by many 

 features of the cleavage, especially by the many departures from the 

 spiral type exemplified in the divisions of the primary trochoblasts 

 and many other cells, and also in the discrepancy between the size of 

 2b and 3b as compared with their sister cells. Further evidence of 

 secondary change is found in the enormous size of 2d and 4d, which 

 have acquired a very large amount of cytoplasm in order to supply 

 material to build up the trunk region, which in the trochophore is 

 ordinarily acquired from the exterior through active feeding. Besides 

 these there is the retarded development of the adult organs, which do 

 not become functional until the animal is nearly ready to hatch, but 

 which must have been primitively functional at a much earlier period. 

 Such organs are the intestine and prototroch. The latter, together 

 with the rosette^ and possibly the perianal band of cilia, which may 

 represent the paratroch, are the sole relics of organs peculiar to the 

 trochophore. It is to be regarded as doubtful if even these would now 

 be recognizable, were it not for the fact that these are also concerned 

 in the formation of adult organs. How quickly such a larval organ as 

 the prototroch may disappear is illustrated in the development of 

 Sternaspis (Child, 1900), where the prototroch has entirely disappeared, 

 though this larval organ is highly developed in related forms. Thus 

 in the light of the cleavage, as well as in that afforded by the more 

 recent work on the anatomy of Dinophilus, Metschnikoff's conjecture 

 appears almost prophetic. Dinophilus probably is to be regarded as a 

 "stationare Annelidenlarva," but one in which the larval stage has 

 become an end stage toward which the development tends and which 

 has become correspondingly modified. 



