1904.] NATURAL SCIENCES OF PHILADELPHIA. 369 



tions to this typical molluscan velum, Ischnochiton 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 Avhich, when viewed in the light of func- 

 tional larval structure, appear as a natural result of the divergent 

 forms of the larvae, these variations having been precociously thrown 

 backward upon the cleaving cells of the ovum. In Aynphitrite, Areni- 

 cola and Clymenella among the Annelids, and Ischnochiton and Trochus 

 representing the more primitive Mollusks, all the primary trochoblasts 

 (la^", 2^2, ^^\ 222, etc.) in all c^uadrants go into the prototroch, while in 

 Nereis the same occurs with the exception of four, which ma}' for all 

 four quadrants be designated la"\; 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 

 (lar^, la^', Ib^^, Ib^'), and the same is true of Planorbis and possibly 

 also of Fiona. Accessory trochoblasts (la^^zi^ la^"2_ ^-j-^j form a part 

 of the prototroch of Ischnochiton in all quadrants, while in Podarke the 

 cells la^222^ lb^222^ 10^222^ 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 lb^2ii jg |j^g "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 1]3'2" 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 ciuartet 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"2^ 2a^2i^ p^c^ j^ Podarke 2a"2 and 2a'2i {^^ A quadrant, and similar 

 cells in B and C, function in like manner, while Ischnochiton shows the 

 same, for 2a"\ 2a"2^ 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 Trochus Robert derives the three cells from the 

 second quartet in A, B and C quadrants (2a"\ 2a"2, 2a^2in^ etc.). 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 arc 

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