1904.] NATURAL SCIENCES OF i'HILADELPHIA. 371 



be observed lying in a row on the rounded edge of the expanding 

 ridge, and though at first this series of cells is indistinctly marked, 

 it continues to increase in definiteness and in the size of its component 

 elements. Figs. 106, 107 and 108 (PI. 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 

 contracted. In fig. 110 it will also be noted that the region just above 

 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 larvae. 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 

 its greatest extent is covered by cells which he posterior to the lateral 

 arms. A posterior cell plate, such as is found in Crepidula, is not here 

 developed, for though doubtless the same cells are present, they have 

 multiplied to a much greater extent than in Crepidula or Planorbis, 

 and form a layer of small cells which are scarcely distinguishable from 

 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 ^\-ith continued 

 growth the apical area becomes pushed forward so that it shortly occu- 



