24 AKT. 4. — N. YATSU : 



laterally, but also posteriorly, as is seen in the figure (PL III., 

 Fig. 44.), and comes to have a narrow space between the posterior 

 edge of the mantle and the body proper. In consequence of the 

 rapid growth of the mantle it becomes somewhat thinned out, 

 leaving the thick part only along the margins. The shell is 

 present at this stage as a very thin cuticle secreted on the entire 

 external surface of the mantle. This is best seen in an enfeebled 

 embryo, in which the mantle is frequently found contracted, the 

 shell being left exposed. The body proper may now be distinctly 

 seen through. On the lateral and posterior sides of the body 

 proper there is seen the thin ectoblast layer separate and at 

 places even having a little space between it and the inner mass. 

 At the median portion of the body proper we see a thick walled 

 sac which constitutes the oesophagus and the stomach 1 (stm.). 

 In the oesophagus the canal is narrow while in the stomach the 

 lumen is somewhat dilated. The space on either side of the 

 alimentary canal, limited externally by the ectoblast layer just 

 described, is filled up by the compact mesoblast cell-masses (msb.) 

 which soon give rise to the coelomic sacs. Up to this time the 

 embryo has rested on the bottom, or probably in nature has 

 been carried to and fro by the currents ; now, however, it begins 

 to swim about freely, widely opening its mantle and protruding 

 its arm-apparatus. 



A little later the arm-apparatus comes to take a nearly 

 pentagonal form, a new pair of protuberances appearing between 

 the tentacle and the first pair of cirri. These new elevations are 

 the Anlagen of the second pair of cirri (cr 11 .). An embryo of 

 this stage is represented in two views : Fig. 46 (PL III.) seen 

 from the anterior and ventral sides ; Fig. 47 from the left and a 



1. Vide footnote under the heading g in Sect. X. 



