l!)()7.] NATURAL SCIENCES OF PHILADELPIirA. 345 



cleterininc that the cerebral gansiUa, and indeed all the gansHa, arise 

 in corres[)ondinj2: parts of the egg in FaUjur and Crepvlula. Such a 

 study shows that amidst all the differences in the mode of develop- 

 ment of these ganglia, there is the most fundamental resemblance in 

 their places of origin in the egg, and that therefore the localization of 

 the germinal sul)stances of the egg in the early cleavage cells follows 

 the same pattern in these two genera. 



5. THE VELUM. 



The velum arises in close relation to the cerebral ganglia, and its place 

 of origin furnishes another striking illustration of what are apparently 

 fundamental differences between Fulgur and Crepidula, l)ut which are 

 in reality fundamental resemblances. In Fulgur the velum is first 

 visil:)le as a dense band of nuclei on the median and posterior sides 

 of the cerebral ganglia (fig. 28, V.). These velar bands are widely 

 separated from one another and are entirely disconnected. In later 

 stages these velar bands more completely encircle the cerebral ganglia; 

 each has somewhat the form of a ]iarabola, one liml) of which runs for- 

 ward in the lip of the blastopore, while the other, which lies posterior 

 to the cerebral ganglion, runs out towards the lateral regions of the 

 embryo and there ends in the general blastoderm (figs. 29, 30). Sub- 

 sequently, \\hen the blastopore narrows and closes, the two limbs which 

 run forward in its lips unite in front of the mouth (figs. 31, 32), while 

 the lateral limb turns forward over the sides of that portion of the 

 embryo which will become the head vesicle (figs. 33, 34). From the 

 posterior side of each band there is given off a branch which runs across 

 the embryo posterior to the mouth and there joins its fellow of the 

 opposite side, thus giving rise to the post-oral band, while that portion 

 of the velum which runs in front of the mouth is the ))re-oral band. 

 Both of these bands were observed and described by Mc>huTi('li (1S8G). 



In subsequent stages the velum is drawn out into a prominent bi- 

 laminar fold (figs. 35, 36). This velar fold or lobe grows out to a very 

 great size, much larger than is shown in fig. 36, the ciliated velar 

 cells being borne around its edge, as in other prosobranchs. By the 

 beating of these cilia it is probable that the embryos are able to move 

 very slowly within the egg capsules, though they never swim freely, 

 the young escaping from the capsules only after the velar lobe has been 

 absorbed. The velum in FuUjur is not a highly sensitive and contractile 

 organ, as in Crepidula and in man}' other forms; apparently, it cannot 

 be retracted, even in its most fully developed stage, and it is needless to 

 say that in the stage shown in fig. 36 there is no cavity into which the 

 velum, the head or the foot could be retracted. 



