372 PROCEEDINGS OF THE ACADEMY OF [April, 



pies the point of greatest anterior extension, while the tip region of 

 the anterior arm through which the velum runs lies ventral to the apex 

 in the direction of the blastopore (figs. 95, 98). At the same time the 

 head end becomes rounded by increased grow^th of the cephalic area. 

 The four original apical cells, as shown in figs. 75 and 76, divide soon 

 after and again at a stage represented by fig. 95, so that this region, 

 which in Crepidula is in the fully developed veliger still marked by four 

 apicals (la"", etc.), here comes to consist of at least twelve very small 

 cells, among which no regularity of arrangement is sufficient!}^ marked 

 to be of value in orientation. These cells are extremely difficult 

 to distinguish from numerous other cells of like form and structm-e 

 which cover the anterior surface of the head vesicle. The apical group 

 continues its for^-ard migration in relation to the larva as a whole and, 

 as it appears, pushes aside some of the cells which have arisen from 

 divisions of the inner and outer basals of the anterior arm, for at a 

 later period (fig. 108) the apical group lies close against the first velar 

 row. Either such a shifting occurs or the basals become involved in 

 the development of the velum. In fig. 108 a row of cells may be 

 distinctly observed in which the nuclei are particularly large, extending 

 laterally from the apical point. My first thought on seeing them was 

 that they were a part of the velum, but after definitely locating the 

 position of the apex and following the later history of the velum, it is 

 clearly seen that this row never enters into the latter structure, but 

 represents in its cell-lineage derivatives of cells of the lateral arms of 

 the cross. No ciliation has been discovered in the apical area, and such 

 structures are certainly not strongly marked, though without examin- 

 ing living material a denial of the possible presence of such structures 

 would scarcely be conclusive. 



Nerve and Sense Organs. 



Cerebral Ganglia. — The cerebral ganglia arise at a stage about 

 corresponding to fig. 105, though they do not become well marked 

 until somewhat later (fig. 108). During this period cells may be 

 seen proliferating inward from the ectoderm of the head vesicle 

 in the two regions which lie lateral from the apical area. A row 

 of cells with large nuclei are at this time plainly visible running 

 laterally from the apex, and it is along the anterior side of these 

 cells that the ganglia first arise. This row has been identified as 

 coming from the lateral arms of the cross, and cells lying between it 

 and the anterior portion of the first velar row are from the same source. 



