DEVELOPMENT OF AGARICIA. 493 



only appearing in the earlier sections. In section 8 the endoderm 

 appears on the ventral side and in section 14 on the dorsal side for 

 the first time. The oesophagus is interrupted first ventrally in section 

 16 and then dorsally in section 19. In the sections the oesophagus is 

 cut obliquely on the ventral side while on the dorsal side it is sec- 

 tioned transversely. In spite of this the oesophagus extends on the 

 ventral side through 12 sections, while on the dorsal side it extends 

 through only 6 sections. 



Although interrupted on the ventral side, the ectoderm of the 

 oesophagus is continuous with the ectoderm of the mesenterial fila- 

 ments of the first two pairs of mesenteries (Fig. 1, C). The mesen- 

 terial filaments of mesenteries / and // of the right side of the larva, 

 (left side of the figure) are fused together as far as section 19, after 

 which they become separated; those of the left side are fused as far 

 as section 21. The mesenterial filaments of the first pair of mesen- 

 teries extend on the right side of the larva as far as section 43; on 

 the left side of the larva as far as section 41. They are larger than 

 the filaments of the second pair of primary mesenteries and there is 

 no evidence of an aboral enlargement as found in the larvae of the 

 later stages. The mesenterial filaments of the second pair of mesen- 

 teries extend on the left side into section 28 and on the right side into 

 section 34. The mesenteries of the first pair extend on the right side 

 of the larva from section 9 to 46, and on the left side from section 1 1 to 

 48. Those of the second pair extend on the right from section 13 to 46, 

 on the left from section 14 to 46. The beginning of the development 

 of the third pair of mesenteries appears on the right of the larva be- 

 tween sections 8 and 31 and on the left between sections 10 and 27. 

 At their oral ends the third pair of mesenteries extends to the ectoderm 

 of the oesophagus. Slight indications of the fourth pair of mesen- 

 teries are found on either side in sections 12 to 18 (Fig. 1, C). Here 

 also the mesentery as defined by mesoglea and muscles extends to the 

 ectoderm of the oesophagus. The distance to which the muscles and 

 mesoglea of the fourth pair of primary mesenteries extend into the 

 endoderm is less than in the third pair. 



Group II. Larva B. 



This larva has six pairs of mesenteries all clearly shown. It was 

 fixed in a saturated solution of corrosive sublimate to which 1% acetic 

 acid was added, and the sections were stained in Heidenhain's iron 



