1904.] NATURAL SCIENCES OF PHILADELPHIA. 365 



the large cell 4b^, which remains for a long time in this position and is 

 closely pressed up into this anterior cone. It may thus be possible that 

 the pointed anterior end of the larva is caused by the shape of the 

 enteron, upon which the outer layer is moulded. 



At first the terminal point of elevation corresponds in position to 

 the tip of the anterior arm, and is thus formed by derivatives of 2b^'^ 

 and neighboring cells. At a somewhat later period the continued 

 growth of the shell gland area pushes the whole apical region forward, 

 so that eventually (figs. 95, 98, 100) this point is carried farther down- 

 ward on the anterior surface. At the same time continued growth 

 has increased the extent of the whole apical region, so that the anterior 

 end becomes more rounded than pointed, and finally (figs. 101, 102), 

 when the veliger stage is just being approached, a broad rounded con- 

 tour characterizes the anterior as well as the posterior end of the larva. 

 It is while these changes are taking place that the first evidence of a 

 distinct velar area appears. Early in this period of forward movement 

 the anterior trochoblasts may be seen to the right and left of the ante- 

 rior end of the forward arm, being distinguished from the derivatives 

 of the second quartet by their smaller size and compact arrangement. 

 They thus, with the tip cell and two other cells behind them (probably 

 Ib^^^S Ib^^^^ derived by transverse splitting of the middle cell), form 

 an irregular row across the anterior edge of the first quartet area 

 (fig. 76). Laterally the posterior ends of this semicircle are joined by 

 cells in the region of the tips of the lateral arms and thus meet the 

 posterior trochoblast groups. These latter have grown larger than 

 their corresponding cells in the anterior quadrants, and so are almost 

 indistinguishable from second quartet elements which lie beneath 

 them. On this account it soon becomes impossible to separate them 

 from these cells, and so at a later period, when the velum in this region 

 becomes marked, I am imable to state how much of it is derived from 

 the trochoblasts, though the httle evidence at hand indicates that they 

 form the largest portion of it. With change of axis the anterior end of 

 the velum is carried forward (PL XXXVII, figs. 95, 98), and the forward 

 end comes upon a level with the antero-ventral surface. A lateral view 

 (fig. 98) shows an irregular row of nuclei (cell outlines are usually in- 

 distinct) running downward and backward from the anterior median 

 point, and becoming lost as it continues posteriorly. This row, which 

 has arisen from the anterior trochoblasts, derivatives of the middle 

 and tip cells of the anterior arm and probably tip cell derivatives of 

 the lateral arms, will be designated V^ Below this band of cells 

 another irregular row may be distinguished composed entirely of second 



