VIII 



PHYLUM MOLLUSCOIDA 



359 



of the ectoderm of the pre-oral lobe, sometimes bearing eyespots, 

 appears to represent the apical plate of the trochophore. At the 

 point where the oesophagus opens into it, the gastric region of the 

 alimentary canal gives off forwards in one species a pair of hollow 

 diverticula, the cells of which contain vacuoles like those of the 

 neighbouring parts of the stomach itself. 



Fig. 288. — Fhoronls, development. A, young larva ; B, larva after the formation of the post- 

 oral circlet of tentacles ; C. larva with commencing pit-like involution ; D, larva with invagina- 

 tion partly everted ; E. invagination completely everted, m. mouth ; an. anus ; iv. involution 

 to form body. (From Balfour's Embryology.) 



The ectoderm of the process on which the anus is situated 

 subsequently becomes involuted to form a deep pit (C, iv), and 

 rudiments of the adult tentacles are formed as a ring of processes 

 at the bases of the larval tentacles. The metamorphosis from this 

 point is completed with great rapidity. The larva sinks to the 

 bottom ; the pit at the side of the anal elevation becomes everted 

 (D), and the alimentary canal of the larva is drawn into it {E), the 

 projection thus formed, which grows out at right angles with the 



