604 



stomach. In some cases, however, they are separated from the wall by 

 a considerable space. 



The transverse section of a larva with 12 tentacles in a plane just 

 posterior to the base of the tentacles but anterior to the mesentery 

 always shows two masses of cells bilaterally placed and closely applied 

 to the mesoderm lining the ventro-lateral somatic wall (Fig. 4 h.c.m). 

 Occasionally I have found cells in these masses situated very close to 

 the mesodermal lining, which were decidedly spindle-shaped in form 

 and whose nuclei resembled those of the cells of the mesodermal lining, 

 both in shape, size and internal structure (Fig. 5 e.c./, Fig. 6). These 

 cells are not very rich in cytoplasm. Most of the cells, however, are 

 almost three times the size of the cells lining the somatic wall, the 

 cytoplasmic part of the cell having increased in size to a greater extent 

 than the nucleus. Most of the nuclei have large deeply staining nu- 

 cleoli (Fig. 5 and 6 m.b.c]. In some specimens, parts of these masses of 

 cells are apparently in the act of wandering across the body cavity to 

 the position the blood corpuscle-masses occupy in the fully formed 

 Actinotrocha. 



I have sectioned some 15 or 20 larvae with 12 or 14 tentacles and 

 with one exception, I have found that when the mesodermal masses 

 are present on the ventral body wall, there are no blood corpuscle- 

 masses present in the larva and that when the blood corpuscle-masses 

 are present, there are no mesodermal masses. In the exception which 

 I have mentioned small blood corpuscle-masses were found applied 

 to the stomach wall and masses of cells bilaterally placed were found 

 on the ventral somatic wall but these cells had already taken on the 

 character of blood corpuscles. 



Ik ed a has described a "mesoblastic cell mass" which he evi- 

 dently considers as giving rise to the adult body cavity and its 

 position is very similar to that of the mesoblastic masses that I have 

 described above. They are both products of the mesodermal lining 

 of the ventral somatic wall and are situated between the plane of the 

 bases of the tentacles and the plane of the somatic insertion of the 

 mesentery between the collar and trunk. Although Ike da does not 

 touch upon the very early origin of the adult body cavity yet it seems 

 probable that he considers it as arising from a single mass of cells. 

 The mesodermal masses that I have described above are paired and 

 bilaterally placed and they are present only in the young larva of 12 

 or 14 tentacles (Fig. 4). Furthermore, in the larva with 12 or 14 ten- 

 tacles there is no sign of the beginning of the adult body cavity. 

 Although these mesodermal masses, which give rise to the blood cor- 

 puscles, according to my observations, have a similar position to the 



