520 A. M. CAiUi SAUNDERS AND MARGAKKT TOOLE. 



Pteropod larvse, and serves to distinguish them from other 

 Molluscan embryos. There may be present only a single 

 liver, as in Fiona, or there may be also a smaller right lobe, 

 as in Aplysia. 



Muscles. — These are two in number, one large and con- 

 spicuous and the other much smaller. 'J'he former makes its 

 appearance at a stage slightly earlier than that represented 

 in PI. 22, fig. 6, when it consists of two or three fibres only, 

 which arise from scattered mesoderm cells descended from 4d. 



Text-fig. 9. 



F M 



ot 



K.2 



R.l 



Similar view of an embryo at a slightly later stage. R.I. Right 

 liver ; other lettering as in Text-fig. 8. 



There is no larval mesoderm in Aplysia derived from the 

 quartettes of ectomeres, which in many cases give rise to the 

 larval musculature. The fibres are at first attached to 

 the body-wall dorsally and to the left of the middle line at 

 about the level of the mantle-cavity (Text-fig. 8). At a 

 later stage they are found to have increased in number and 

 to be attached further back and rather more ventrally (Text- 

 fig. 9), while in the free-swimming larva their attachment 

 to the body-wall is posterior, but slightly to the left of the 

 middle line and still nearer the ventral surface (Text-fig. 10). 



