1904. ] NATURAL SCIENCES OF PHILADELPHIA. 371 
ridge, and though at first this series of cells is indistinetly marked, 
it continues to increase in definiteness and in the size of its component 
elements. F igs. 106, 107 and 108 (Pl. XXXV) show successive 
Stages in the elaboration of these large heavily ciliated cells of the 
velar edge, and sections, as figs. 91 and 92 (Pl. XXXII) in particular, 
Show the great increase in size which now marks them. 
Coincidently occurs the expansion of the velar lobes to form the 
broad wings or velar folds which characterize the functional larva 
at the time it becomes free-swimming. As the velar area expands it 
becomes deeply notched below where the lobes of the Opposite side 
rise to meet over the mouth, and this growth in length and breadth is 
marked on the dorsal side as well. Figs. 109 and 110, side and dorsal 
views of the same veliger, show the condition of development of the 
velum just before the larva breaks from the egg capsule, though in these 
drawings from fixed material the velum is of necessity considerably 
be observed lying in a row on the rounded edge of the expanding 
the mouth has grown out into a projecting process, and it is upon this 
area that the former apical point (animal pole) lies. 
Head Vesicle. 
The Head Vesicle of Fiona reaches its greatest prominence at a stage 
shown in fig. 104 and slightly older larve. Somewhat later (figs. 105, 
106) it becomes actually larger, but relatively smaller when compared 
with the larva as a whole, and has also become greatly involved in the 
formation of the velar lobes. It is composed of cells of the first quartet 
lying within the trochoblasts and ends of the arms of the cross, and 
those in front or at their sides. Neither is an apical cell plate demar- 
kated in the region corresponding to the location of that Structure in 
Crepidula, the cells in front of the apex being all of similar size and 
Seemingly without regularity of arrangement, so that it is with the 
greatest difficulty that the apex can be located among the large number 
of small cells of equal size by which it is Surrounded. As has been 
described before, the point of greatest forward extension lies first in the 
region of the tip of the anterior arm of the cross, but with continued 
growth the apical area becomes pushed forward so that it shortly oceu- 
