ON THE DEVELOPMENT OF LINGULA ANATINA. 21 



are obscured by pressure. Here we see distinctly the connection 

 of the stomodœum (stmd.) and the archenteron (arch.). 



The depressions on the mantle fold become deeper and are 

 no longer confined to the anterior surface but pass around two 

 sides. The ring-shaped mantle fold is thus cut up into two lobes. 

 The cutting line finally extends all around the posterior wall of 

 the embryo. Concurrent with this change the mantle fold which 

 has thus been divided, and may now be called the mantle lobes, 

 rapidly increases in size, especially at the anterior margin, so 

 that the lobes come to assume a nearly circular form. Fig. 40. 

 is a side view of such an embryo. In this a large central mass 

 composed of the body proper and the arm-apparatus, and two 

 widely separated mantle lobes are distinctly visible. 



An embryo of the same stage is represented in a dorsal view 

 in Fig. 39. (PI. III.). Here we see a very large arm-apparatus 

 with a rough surface ; the dorsal mantle-lobe is in full view, while 

 the ventral is concealed by the arm-apparatus. On applying 

 slight pressure from the dorsal side upon another embryo of a 

 similar stage (PI. III., Fig. 41.) the tolerably spacious archenteron 

 (arch.) and the segmentation cavity (sg. cv.) became visible. In 

 this the arm-ridge 1 is found to be densely covered by cilia, whose 

 slow movement can be distinctly seen through the vitelline mem- 

 brane. The formation of the mantle lobes is thus accomplished by 

 the cutting of the mantle fold and the posterior wall by a furrow, 

 and by the anterior and lateral growth of the mantle. This 

 process must not be confounded with the later growth of the 

 circular mantle lobes in its diameter. 



The next changes consist chiefly in the flattening of the 



1. In the embryo the arm-ridge is exceptionally small. Normally it bulges out a great 

 deal above the mantle lobe (Cf. Figs. 09 and 40). 



