370 PKOCEEDLXGS OF THE ACADEMY OF [April, 



added, so that the second row contains " probably a few cells of the first, 

 many of the second and possibly a few of the third quartet". The 

 velum of Planorbis is rudimentary in structure but shows the same gen- 

 eral type of development as Crepidula, and here in like manner second 

 quartet cells are added. The tip cells of the lateral arms, according 

 to Holmes, do not enter the prototroch, but cells of the same series 

 below them function in this manner. In the anterior region l)oth tip 

 cells and those lying beneath them from the second c^uartet enter into 

 the prototroch. 



From this short comparison of the lineage of the trochal area in 

 Annelids and Mollusks, it will be seen that as in the functional larval 

 form the typical molluscan velum shows greater anterior development 

 than the prototroch of Annelids, so also cells taken from the segmented 

 egg to complete the velum in this region exceed in number those des- 

 tined to form a similar area of the anzielidan trochophore. To do this 

 the second quartet has become greatly encroached upon in furnishing 

 necessary building material for this structure in those Mollusks whose 

 larvae show strong anterior velar development, and in Crepidula the 

 third quartet also possibly becomes involved. It is natural to conclude, 

 as indeed the facts show, that those Mollusks which in the structure of 

 their larval prototrochs show great similarity to the homologous struc- 

 ture of the Annelid trochophore, will exhibit a similar lineage of the 

 -cells constituting the larval organs compared — examples, Ischnochiton 

 and Trochus. 



Later Velar Development. — With continued invagination of the 

 stomodseum and constriction of the foot, the velar area, which has 

 thus far been marked only by an irregular double row of cells 

 extending around the anterior half of the head vesicle and losing 

 itself in the posterior portion of that larval organ, becomes more 

 prominent and takes on the bilobed outline so characteristic of 

 the anterior end of veliger larvae. At first the velar lobes are 

 merely rounded swellings gradually rising from the upper sides 

 of the head vesicle and curving around, downward and inward 

 toward the stomodaeum (fig. 105). The cells in this region do not 

 as yet exhibit that differentiation which later marks the promi- 

 nent ciliated margin from the undei'lying region. But as the lobes 

 begin to constrict beneath and become more prominent (fig. 106), those 

 cells which lie on their most peripheral surface show marked increase 

 in size, and the ciliation which hitherto has been uniform and weakly 

 developed becomes more prominent in these cells. Thev may now 



