1904.] ' NATURAL SCIENCES OF PHILADELPHIA. 369 
tions to this typical molluscan velum, Jschnochiton and Trochus for 
examples, in which the trochal ring is as complete as among the Anne- 
lids. Returning now to the developmental history of the two groups 
certain variations are found which, when viewed in the light of func- 
tional larval structure, appear as a natural result of the divergent 
forms of the larvee, these variations having been precociously thrown 
backward upon the cleaving cells of the ovum. In Amphitrite, Areni- 
cola and Clymenella among the Annelids, and Ischnochiton and Trochus 
representing the more primitive Mollusks, all the primary trochoblasts 
(la, 2%, 221, 222 etc.) in all quadrants go into the prototroch, while in 
Nereis the same occurs with the exception of four, which may for all 
four quadrants be designated 1a”; these are not functional in this 
manner, but are pushed inward and form part of the cephalic vesicle. 
In Crepidula only the anterior trochoblasts help form the preoral velum 
(la”, 1a?!, 1b”, 1b*!), and the same is true of Planorbis and possibly 
also of Fiona. Accessory trochoblasts (la, la”, etc.) form a part 
of the prototroch of [schnochiton in all quadrants, while in Podarke the 
cells la”, Ib?”?, 1c”, corresponding to three of the above series, aid 
in the formation of the prototroch (‘secondary trochoblasts” of Tread- 
well). In Planorbis Holmes finds that the cell 1b’ is the “anterior 
median”’ cell of the prototroch, but does not find similar conditions in 
any other quadrants. None of these elements which are, of course, 
derivatives of the annelidan outer intermediate or molluscan middle 
cells (with the exception of 1b" of Planorbis, which comes from the 
inner basal) are found in the antero-lateral portion of the prototroch 
of Amphitrite, Arenicola, Clymenella and Nereis. In all the above 
forms except Nereis elements from the second quartet are also added 
to the prototroch, and these may be designated with Treadwell ‘“ter- 
tiary trochoblasts’. In Amphitrite, Arenicola and Clymenella the 
prototroch is increased in A, B and C quadrants by the cells 2a", 
2a'*, 2a, etc. In Podarke 2a'” and 2a)! in A quadrant, and similar 
cells in B and C, function in like manner, while Jschnochiton shows the 
same, for 2a, 2a'”, etc., enter the prototroch from the anterior and 
lateral quadrants (“‘secondary trochoblasts” of Heath). Of Hy- 
droides Treadwell says: ‘Cells are added from the lower hemisphere’’. 
For the prototroch of 7’rochus Robert derives the three cells from the 
second quartet in A, B and C quadrants (2a!!, 2a"?, 2a"! ete.). Com- 
ing to those Mollusks which possess a typical veliger, more cells are 
found to be contributed by the second quartet, particularly in the 
anterior quadrants. In Crepidula the tip cells of the anterior and 
lateral arms go into the first velar row, while below numerous cells are 
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