510 A. M. (JAlUi SAUNDERS AND MARGARET TOOLE. 



simple ring of cilia round the anterior end of the body (PI. 

 22, lig. 4). As the anterior end, however, grows out within 

 the velar area, and then becomes flattened and expanded late- 

 rally, the circular shape of the ciliated band is soon lost, and 

 the latter comes to surround the widely extended anterior pro- 

 longation of the body (PL 22, fig. ()). The velum then 

 becomes notched in the mid-dorsal line and bilobed, but the 

 latter characteristic is not so well marked as is generally the 

 case in Opisthobranch larva). 



In the free-swimming veliger the full extension of the velum 

 is reached, but it can always be contracted completely within 

 the shell. The cilia are long and prominent. Inside the 

 circle of these cilia-bearing cells is a second row of cells of 

 rather larger size with three or four cilia each, and in the 

 middle, a cell with a single long and prominent ttagellum. 



The Foot. — The rudiment of the foot is at first broad and 

 blunt, projecting from the ventral sui-face of the embryo 

 between the blastopore and the anal cells. There is no sign 

 of a division into two, as has been described in early stages 

 of Patella. Between the stages represented in PI. 22, figs. 4 

 and 5, the foot has undergone considerable change in shape, 

 becoming elongated in an antero-posterior direction and 

 flattened dorso-ventrally, and the operculum has been secreted 

 on the lower surface. In the free-swimming larva it is still 

 longer and covered with short cilia, and the operculum is 

 capable of closing the opeuing of the shell completely when 

 the animal is retracted. 



The She 11 -gland. —In PI. 22, tigs. 2 and 8, the shell- 

 gland is invagiuated to form a narrow pit. It soon afterwards 

 becomes everted, and fig. 4 of the plate shows the posterior 

 end of the embryo covered with a thin shell. The cells that 

 were invaginated now form a cap, which secretes the shell, 

 the edge of the former becoming the edge of the mantle. 

 The mantle-cavity in the free-swimming lai'va is fairly deep, 

 and into it on the right side open both the anus and the 

 secondary kidney. 



The Shell. — This is secreted directly the shell-gland is 



