THE RISE OP THE LARVA, ETC. 



125 



the middle of the blastopore and the opposite pole at which later the 

 apical plate develops. Through the appearance of the mesoderm, 

 the larva becomes bilaterally symmetrical. The blastopore soon 

 changes its position, shifting forward on the ventral surface, as a 

 consequence of the active growth of the dorsal surface. The rudi- 

 ment of the velum which was indicated at the blastula-stage has now 

 become more distinct (Figs. 49 and oO). In later stages, the dis- 

 placement of the blastopore becomes much more striking, and recalls 

 the condition already described in connection w T ith Dentalium (Figs. 

 34 and 36, p. 91). The blastopore, during this process, changes 

 from its round form and becomes slit-like (Fig. 51 B). At its 



Fig. 51. — Trochophore larvae of Patella at two different stages (after Patten). Iu .1, 

 the two lateral pedal swellings can be seen near the circular blastopore. In B, the 

 blastopore appears lengthened. Near it can be recognised the rudiments of the two 

 mesoderm-bands, and behind it the anal ciliated tuft. 



posterior end, two cells are distinguished by their special size. They 

 soon become covered with cilia (Figs. 51 and 52), and may well be 

 compared to the anal cells of other Gastropods which will be described 

 later (p. 142). The slit narrows and closes in from behind forward. 

 The anterior part of the blastopore remains in the form of a round 

 pit in the position of the future mouth ; later, the blastopore is 

 carried inwards by a depression of the ectoderm, the stomodaeum, 

 which occurs at this point. This depression represents the rudiment 

 of the oesophagus (Fig. 50 B), the blastopore persisting as the opening 

 betw r een the stomach and oesophagus. Out of this solid mass of 

 cells, which still represents the entoderm, the enteron forms later 



