368 PROCEEDINGS OF THE ACADEMY OF [April, 



quartet, though he adds that at a later period cells are joined to the 

 prototroch from below, the lineage of which is obscure. 



In Ischnochiton, the larva of which is, in its velar aspects, remarkably 

 like the trochophore of Annelids, Heath finds that the prototroch is 

 composed of trochoblasts, of ''accessory trochoblasts" (derived from 

 the original basal cells of the molluscan or intermediate girdle cells 

 of the annelidan cross) of the tip cells in the anterior and lateral arms, 

 while in the posterior arm the tip cells go into the ventral plate, the 

 gap in the trochal ring being there bridged by derivatives of the median 

 cell of that arm of the cross. Thus in this annelid-like form of larva 

 none but derivatives of 2a", 2b" and 2c" from the second quartet 

 form the trochal ring. 



The prototroch of Trochus (Robert) is composed of twenty-five 

 cells, sixteen of which comprise the trochoblasts, six represent the 

 divided tip cells of A, B and C quadrants, while the other three are the 

 cells 2a, b, c^^"^ A very exact and close comparison may here be 

 made with the prototroch of the Annelids A^wphitrite , Arenicola and 

 Clymenella, particularly with the former, for, as Robert says, "Vingt- 

 deux ont indetiquement la meme origine et la meme disposition que 

 celles de Amphitrite; le trois autres (2a, b and c^"^) sont des derives 

 des cellules correspondantes de la meme Annelide.'" 



Among Annehds Wilson has found that the prototroch of Nereis 

 arises entirely from twelve of the sixteen primary trochoblasts, there 

 being no contribution from the second quartet. All sixteen of the 

 primary trochoblasts enter the prototroch of Amphitrite and Clymenella 

 (Mead), as is also the case with Arenicola (Child) and Podarke (Tread- 

 well). Regarding the close resemblance between the trochophore of 

 Ischnochiton and those of the Annehds, Heath says: "The origin, 

 development and fate of these cells (primary trochoblasts) is pre- 

 cisely similar to the primary trochoblasts in Ischnochiton. The second 

 quartet in Amphitrite, Clymenella and Arenicola furnishes three cells 

 in each quadrant except the posterior, which enter the prototroch. 

 Two of the three are homologues of the divided tip in Ischnochiton, 

 while the third corresponds to a post-trochal cell". 



If now we compare the derivation and ultimate structure of the 

 annelidan prototroch with the typical molluscan velum some inter- 

 esting causal relations appear. At the time of its functional activity 

 the prototroch of Annelids is apparently a radially symmetrical struc- 

 ture. Among the Mollusks we find, as a rule, a velum strongly devel- 

 oped anteriorly, with a considerable area of weakly ciliated ectoderm 

 between the ends of its posterior arms. There are numerous excep- 



