ON THE DEVELOPMENT OF LINGULA ANATINA. 



33 



a). Giving rise to the coelomic sacs. 

 ß). Filling up the arm-sinus as mesenchyme cells. 

 10). The body cavity is formed after the schizocœlic type. 

 11). The ectoblast gives rise to four structures : 

 a). The mantle. 

 ß) . The outer epi- 

 thelium of the 

 arm-apparatus. 

 }-). The anterior 

 half of the 

 oesophagus, 

 o). The ectoderm 

 of the lateral 

 body wall. 

 12). The mantle is 

 formed at its first appear- 

 ance as a ring-like fold, 

 which afterwards is divided 

 into two mantle-lobes. 



13). The shell is se- ( 

 creted as a circular lamella 

 folded double along one of 

 its diameters. 



14). The arm-appara- 

 tus is formed at first as the 

 arm-ridge, which on increas- 

 ing in size comes to be 



. 1 .. (Sagittal section). 



provided With the tentacle 7. Embryo with shells. (Sagittal section). 



8. The same. (Frontal section). 



and pairs of cirri. 



15). A new pair of cirri is always added next to the tentacle. 



hell. 



— mes-enteric 

 cell-mass. 



mantle-fold. 



-- archenteron. 



elntacle. 



-body 

 cavity. 



Ectoblast-crossed. 



Mcsoblad and Mesenchyme-dodcd. 



1. Blastula. 



2. Gastrula. 



3. The same further advanced. 



4. The same cut along x y (The archenteron ob- 



literated). 



5. Embryo at the beginning of the mantle-fold 



formation (Sagittal section). 



6. Embryo, in which the stomodœum is formed. 



