630 HAROLD HEATH, 



valve increasing in extent. During the next day the shell advances 

 still farther, the velum is cast off and within a week the lower sur- 

 face of the head vesicle becomes flatter giving the embryo in side 

 view an appearance shown in Fig. 55. Shortly after this time anterior 

 prolongations of the mantle furrow appear in the head vesicle below 

 the shell (Fig. 56). It is shallow at first but gradually deepens 

 bringing into prominence the area which becomes the proboscis. At 

 this stage the mouth lies posterior to this area but gradually pushing 

 forward it comes to lie in the centre of the developed snout. At this 

 time the embryo appears as in Fig. 58. The rounded convex ap- 

 pearance of the proboscis rapidly disappears and the condition shown 

 in the figure is reached. 



Thus the head vesicle becomes transformed into part of the first 

 valve of the shell, the mantle and mantle furrow of the same region 

 and the proboscis, and as will be remembered these are thus derived 

 from the first quartette of ectomeres. 



In the trunk region few changes take place ; the mantle furrow 

 becomes deeper and the foot more differentiated. When the anus 

 breaks through I cannot say definitely for Ischiochiton, but I know 

 that in the 15 day embryo this process has not occurred. In another 

 species {Trachydermon) however this process takes place at about the 

 time corresponding to the 15 day stage of Ischnochiton. 



As to the gills I may say that they arise as minute papillae in 

 the mantle furrow and at a relatively late period, several days beyond 

 the formation of the proctodaeum. 



3. The Foot. 



Shortly after the blastopore comes to be situated immediately 

 posterior to the velum, the foot, more or less quadrangular in outline, 

 arises as a median undivided protuberance on the ventral surface 

 (Fig. 51). Faintly demarcated at first it gradually grows in prominence 

 by a deepening of the surrounding groove, and as this process is 

 taking place contractile movements commence to manifest themselves 

 along its anterior border. Rapidly they extend themselves to all parts 

 of the organ and about the close of the free- swimming period the 

 foot reaches its maximum of contractility, changing its shape with a 

 rapidity that is never again met with in its history. 



It is composed of high columnar cells, all of about the same size, 

 which become clothed with fine cilia before the embryo leaves the 

 chorion. The exact origin of the cells is open to some doubt, yet 



