370 PROCEEDINGS 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 both tip 
cells and those lying beneath them from the second quartet enter into 
the protdtroch. 
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 
ege to complete the velum in this region exceed in number those des- 
tined to form a similar area of the annelidan trochophore. To do this 
the second quartet has become greatly encroached upon in furnishing 
necessary building material for this structure in those Mollusks whose 
larve 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 T'rochus. 
Later Velar Development.—With continued invagination of the 
stomodzeum 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 larve. 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 stomodeum (fig. 105). The cells in this region do not 
as yet exhibit that differentiation which later marks the promi- 
nent ciliated margin from the underlying 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. They may now 
