198 ARTHUR WILLEY. 



just in front of the one marked '^ second primary slit.^^ In 

 fig. 8 there are indications of fourteen primary slits ; the 

 twelfth is, however, very small, and the thirteenth and four- 

 teenth are on the verge of closure. 



In both fig. 9 and fig. 10 the twelfth primary slit is shown 

 in course of closure. 



Fig. 10 is quite an exceptional view of a larva from the 

 ventral or ventro-lateral aspect, obtained by the larva getting 

 fixed on its back between slide and cover-glass, as a result of 

 its struggles to get free. Such a complete view as is here 

 represented can only be got very rarely, and when it does 

 happen it lasts but a very few minutes, as the larva either 

 speedily rights itself or dies. 



The shape of the primary and secondary slits has been 

 already described. The tongue-bars of most of the latter have 

 fused with the ventral borders of the slits, but not one has 

 done so in the case of the primary slits. It should be noticed, 

 also, how the row of primary slits curves round to the middle 

 ventral line behind ; the twelfth is nearly closed, the eleventh 

 would close later on, while the tenth might or might not close 

 later : judging from the shape and size of it, one may say 

 that the balance of probability is in favour of its not closing. 

 In some of the primary slits there is a small up-growth from 

 the ventral border of the slit, pointing towards and subse- 

 quently destined to fuse with the down-growing tongue-bar. 

 A.11 the slits are of course seen through the transparent 

 atrial wall. The endostyle is seen anteriorly between the 

 primary and secondary slits, and the buccal cirri are also well 

 shown. 



The mouth of this stage is shown in fig. 8. The lower por- 

 tion of the oral hood, carrying the buccal cirri, is now more 

 or less continuous with the upper portion into which the cirri 

 have not yet extended ; but, at the point of junction, a ridge 

 is formed under which the cartilaginoid tissue eventually forces 

 its way (cf. figs. 12, 13). The upper and lower portions of the 

 oral hood become respectively left and right. 



Thus, while the right and left halves of the oral hood are 



