112 CONK LIN. [Vol. XII L 



Even in the oldest stages which I have drawn, Fig. 104, there 

 is no indication of a pedal ganglion, except a few cells which 

 lie between the ectoderm and the otocysts, and which have 

 evidently come from the connectives. 



(f) The Apical Cell Plate. — The origin of the apical plate 

 from seven cells of the anterior arm of the cross has already 

 been described (p. 91). These cells become covered with a 

 coat of fine cilia, and form a very definite plate, extending from 

 the apical organ to the velum. They are especially notable in 

 that they remain very large, and do not divide during a period 

 when all the surrounding cells are dividing rapidly, and are 

 relatively quite small. In later stages. Figs. 65 and 6^, the 

 anterior apical cell is crowded forward between the basal cells 

 of the anterior arm, and in still later stages, Fig. 79, the inner 

 basal cells divide, and two other large cells (probably derived 

 from the outer apicals) are found on each side of the four 

 apicals. I have followed this plate of cells through to the free 

 veliger stage, but have not determined its ultimate destination 

 in the post-larval period. 



A plate of such definite and peculiar structure must have, I 

 think, some special significance, and I believe it deserves to 

 rank as a larval organ, though I do not know what function it 

 subserves. I have called it the "apical plate " because of its 

 position and structure, and have not intended thereby to assert 

 its homology with the " Scheitelplatte " of the annelid trocho- 

 phore. Its resemblance to the " Scheitelplatte " is suggested 

 by its position, by its being covered with cilia, and by its rela- 

 tion to the apical thickening, which forms the apical organ. It 

 is unlike the " Scheitelplatte," as described by Hatschek ('78), 

 in that it lies chiefly in front of the apical pole, and does not 

 form the supraoesophageal ganglia. On the other hand, as 

 has been pointed out (p. no), there is no doubt that the 

 "cephalic neural plate" of Nereis corresponds in position, in 

 destiny, and probably in cell origin, to the cerebral ganglia, 

 commissures, and apical organ in Crepidula. 



(g) The Preoral Vehmi {Prototroch). — The velum is first 

 plainly recognizable at a comparatively late stage, Figs. 6$ et 

 scg., and at a time when there are several hundred cells present. 



