DEVELOPMENT OF AGAKICIA. 497 



Number of mesentery / II III IV V VI 



Right side of larva 



Oral end section No. 



Aboral end section No. 

 Left side of larva 



Oral end section No. 



Aboral end section No. 



In section 49 mesenteries / and II of the right side are united at their 

 internal ends. Only mesenteries / to IV are complete. 



Group III. Larva F. 



This larva (text, Fig. 2) is not so much contracted along its oral- 

 aboral axis as the one last described, and the oesophagus is only slightly 

 invaginated. It was fixed in a saturated solution of corrosive subli- 

 mate and the sections stained in Mallory's phosphotungstic acid 

 haematoxylin. The oral end of the larva was cut first. The sections 

 are almost exactly transverse. The oesophagus appears as a com- 

 plete circle of ectoderm in the third section, the first two sections 

 showing the dorsal side only. The sections are 6 /x thick and there 

 are 86 of them. Of the sections, Figure 2, A-I, shows numbers 9, 13, 

 18, 21, 33, 45, 61, 71 and 74 and Plates 3 and 4 show photomicro- 

 graphs of numbers 12, 17, 61 and 73. 



The oesophagus is elliptical in cross section, the long axis being 

 dorso-ventral. It ceases on the dorsal side at section 9 (Fig. 2, A). 

 Below this its cross section is U-shaped until section 13 (Fig. 2, B) 

 is reached. Below section 13 it is interrupted on the ventral side also. 

 The invaginated ectoderm of the right side of the larva is joined by 

 the first, second and third mesenteries, that of the left side is joined 

 by the first and second mesenteries (Fig. 2, C). 



Below section 20 the ectoderm is continued on both sides as the 

 separate mesenterial filaments of the first and second pairs of mesen- 

 teries. The filaments of the first pair extend on the right as far as 

 section 70 and on the left as far as section 73, which is practically the 

 whole extent of the gastrovascular cavity. The filaments of the second 

 pair extend on both sides as far as section 56. The mesenterial fila- 

 ments are enlarged as follows: those of the first pair of mesenteries 

 on the right between sections 49 and 66, on the left between sections 

 52 and 70, those of the second pair of mesenteries on both sides between 

 sections 32 and 54 (Fig. 2, E-H). 



The six pairs of primary mesenteries extend practically the whole 



