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 [schnochiton, 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@444. A very exact and close comparison may here. be 
made with the prototroch of the Annelids Amphitrite, Arenicola and 
Clymenella, particularly with the former, for, as Robert says, ‘‘ Vingt- 
deux ont indetiquement la méme origine et la méme disposition que 
celles de Amphitrite; le trois autres (2a, b and c™"") sont des derives 
des cellules correspondantes de la méme Annelide.”’ 
Among Annelids 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 Annelids, 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- 
