No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 42 1 



lie at nearly the same horizontal level, the originally upper cell 

 lying in front. The tip cells of the lateral arms lie immediately 

 behind the anterior trochoblasts, but do not, as I formerly sup- 

 posed, form a part of the prototroch, but enter into the forma- 

 tion of the head vesicle. They mark, in fact, the transition 

 between these two structures, and might be considered as 

 greatly enlarged velar cells. The chief differences between the 

 first row of velar cells in Planorbis and Crepidula are that in 

 the former the lateral tip cells become greatly enlarged and do 

 not divide, and the anterior tip cell divides but once, the daugh- 

 ter-cells becoming pushed apart by one of the cells of the first 

 quartette. 



The lower row of cells in the prototroch is derived from the 

 second quartette. Conspicuous among these are the two large 

 cells lying below the median portion of the upper row. They 

 are symmetrically placed on either side the median line. The 

 two cells originally lying above these, 2(^''^'' and 2<^^'', have 

 been forced aside by the forward rotation of the upper pole, 

 so that the tip cells, 2/^'''' and 2<^''''^ come to lie next to the 

 cells 2<5'''"'' and 2(5^' ■^ The cells of the lower row at the sides 

 are small, and are added to the prototroch at a later stage, 

 when the number of cells in the regions from which they arose 

 was so great that it would be very difficult to trace their lineage. 



The Shell Gland and the Foot. 



The shell gland makes its appearance some time after the 

 closure of the blastopore. A very early stage in the develop- 

 ment of this structure is shown in PI. XXI, Fig. 51. It is 

 located a short distance behind the tip of the posterior arm of 

 the cross, in a region which is formed from the cells of the 

 second quartette. It is derived, doubtless, from derivatives 

 of 2^''^ and 2^^*. It forms a tolerably deep invagination, the 

 cavity of which becomes almost entirely obliterated. 



The foot arises as a protuberance behind the mouth. The 

 two halves are separated by a row of clear cells extending back- 

 ward some distance from the definitive mouth — a fact which 

 may or may not indicate a double origin of this organ. A very 



