464 R. M. PACE. 



we have seen, this structure is a true stomodseum, and from 

 the above account it will be evident that the alimentary 

 apparatus is much better developed in Flustrella than 

 Prouho had supposed to be the case, in that it really com- 

 prises both stomach and pharynx, though both it is true are 

 of a rudimentary character. 



It will be remarked, when viewed in relation to the 

 vestigial character of the stomach, that the considerable 

 development of the pharynx is somewhat surprising ; but 

 may it not be that the pharynx has, in accordance with the 

 exigencies of larval life, assumed another function? Mention 

 has already been made of the globules which are present in 

 the vacuoles of the cells of the pharynx. At about the time 

 that these disappear, the larva becomes freed from the vitel- 

 line membrane, and at the same time the slimy, mucus-like 

 substance already noted becomes very abundant around the 

 embryos. Possibly, therefore, the pharynx has assumed a 

 glandular function, and it may well be that this mucus-like 

 substance has been derived from the globules previously con- 

 tained in the pharyngeal cells. Attempts to prove, by treat- 

 ment with Mayer's mucicarraine, that the globules in the 

 pharynx and internal sac really consist of mucus, have so far 

 given negative results; and this point must, therefore, for the 

 present remain unsolved. The suggestion here put forward 

 may also afford an explanation of the existence of the drops 

 previously noticed as exuding from the cells of the internal sac. 



In Pi. 25, figs. 63 b, 64, are shown two bands of tissue 

 marked '^ which appear to be budding off from the sides of 

 the mesendodermic mass. Their significance will be discussed 

 later. 



The Mature Larva. 



In PI. 25, figs. 65 a-c, the larva is represented at a stage 

 shortly anterior to that at which degeneration commences. 

 It is now enclosed in a chitinous bivalve shell, and has 

 escaped from the vitelline membrane. From this point 



