150 WILLIAM BATESON. 



this period a posterior transverse ring of cilia is found. The 

 body elongates, and becomes marked out into regions, by the 

 appearance, first of an anterior groove situated nearly in the 

 middle line, and secondly of a posterior groove shortly behind it. 

 The area in front of these grooves is prseoral, and is destined to 

 form the proboscis, while the region between them constitutes 

 the collar. 



The invaginated hypoblast at first forms a simple lining to 

 the cylindrical body. That portion of it which lies in front of 

 the anterior groove then segments off from the rest, forming an 

 anterior unpaired body cavity. This cavity sends back a horn 

 for a short distance on either side of the gut. 



In the region of the collar a pair of splits occur in the hypo- 

 blastic walls, whose cavities open into the archenteron. The 

 cells forming their walls then separate themselves from the 

 remaining hypoblast as a pair of closed pouches placed symme- 

 trically, one on each side of the body. 



A pair of archenteric diverticula are also formed in the region 

 of the trunk, which, on losing their connection with the gut, 

 persist as another similar pair of body cavities. 



The nervous system is formed by a segregation of epiblastic 

 cells in the dorsal middle line of the collar, forming a cord 

 lying immediately beneath the skin, continuous with it at both 

 ends of its course. 



The mouth is a small pore in the ventral middle line placed 

 in the anterior transverse groove. 



The larva is always opaque, and on being hatched creeps 

 about in the muddy sand which the parents inhabit, at no time 

 leading a free life at the surface comparable with that of 

 Tornaria. 



I hope shortly to describe the remaining structures existing 

 at Stage F, and to give some account of the older larvae, and of 

 certain points in the anatomy of the adult. 



It may, perhaps, be convenient briefly to allude to some of 

 the latter which are of value in interpreting the facts already 

 given. Of these, three are of especial importance. 



1. The nervous system (Spengel (7) ) in the adult is made 



