178 WORMS. 



and secondarily the nervous system, the greater part of the 

 nephridia, and the setae, while it is also tucked in to form 

 the beginning and end of the gut. The endoderm forms 

 the internal lining of oesophagus, crop, gizzard, and the 

 intestine, except the last little bit which is due to the ecto- 

 derm. Everything else arises from the mesoderm. But 

 this layer requires further analysis. 



The mesoderm begins with the two primary mesoblasts 

 already described. These multiply and form mesoderm 

 bands, which, insinuating themselves between ectoderm and 

 endoderm, proceed to surround the gut. At the same 

 time, some of the mesoderm cells become migratory, wander 

 on to the head, and also surround the gut, before the final 

 trunk musculature is completed. The migratory mesoblasts 

 of the trunk appear to form a special larval musculature 

 precociously developed, in order to enable the embryo to 

 manage the enormous mass of albumen (absorbed from the 

 capsule) with which its body is distended. The mesoderm 

 bands grow in strength, and form a complete ring encircling 

 the archenteron. 



Origin of the body-cavity. The mesoderm bands, growing 

 in strength, become two-layered. These two layers separate, 

 the inner (splanchnic) cleaving to the gut, the outer (somatic) 

 clinging to the body-wall. The space between them is the 

 body-cavity or ccelome. But as the separation of somatic and 

 splanchnic layers takes place, partitions are also formed 

 transversely, to become the septa which partition off the 

 body-cavity into a series of segments. The cavity of 

 the pre-oral segment or prostomium differs somewhat from 

 that of the others, being from the first unpaired, instead 

 of including two lateral cavities one on each side of the 

 gut. 



As to the blood-vessels, the ventral or sub-intestinal appears 

 first, as a space between the wall of the archenteron and the 

 underlying mesoderm ; the dorsal vessel has a double origin, 

 arising from the fusion of two lateral vessels which develop 

 like the ventral. The important point is, that the blood- 

 vessels are at first long lacunar spaces, which gradually 

 acquire definite walls. By and by the " hearts " and other 

 complications in the vascular system appear. 



The reproductive organs, though probably arising from 



