DEVELOPMENT OF AGARICIA. 501 



to 41. The filaments of the same side are not fused, as in some previ- 

 ous larvae, except that on the left side they are united at their origin, 

 (section 23). The enlargement of the filaments of the first pair of 

 mesenteries reaches from section 29 to 38; the sections of the second 

 pair are of almost uniform size throughout their length. 



The mesenteries extend practically the entire length of the larva; 

 their positions are shown in the following table: 



Number of mesentery / II III IV V VI 



Right side of larva 



Mesenteries I to IV are the only complete ones. 



Group IV. Larva 1. 



This larva is slightly larger than any of the previous ones, and shows 

 an advance in the development of the primary mesenteries and of the 

 mesenterial filaments. It was fixed and stained like larva G, and 

 then cut into 133 transverse sections, each 5 fj. thick, the aboral end 

 being cut first. Figure 4 (A-F) shows the 15th, 19th, 23rd, 25th, 73rd 

 and 93rd sections. Photomicrographs of sections 19 and 26, are 

 shown on Plate 5 and of 73 and 93 on Plate 6. 



The oesophagus is slightly more invaginated than in larva B. It 

 is oval in cross section (Fig. 4, A) from the oral end to section 17. In 

 section 17 it is U-shaped, the ectoderm being interrupted on the dorsal 

 side. This is also the case in section 19 (Fig. 4, B). Beyond this 

 section the ectoderm is interrupted also on the ventral side (Fig. 4, C). 

 On the ventral side of the oesophagus in sections 16 to 19 a diverti- 

 culum of the gastrovascular cavity extends forward in the region of 

 the third pair of mesenteries (Fig. 4, B). The mesenterial filaments 

 of the first and second mesenteries (Fig. 4, D) are fused on the right 

 (left as seen in the figures) as far as section 29, on the left as far as 

 section 38. 



The mesenterial filaments of the first pair of mesenteries extend on 

 both sides into section 112. The transverse sections of these fila- 

 ments are smaller than those of the filaments of the second pair of 



