174 INVEETEBEATA chap. 



125 A). By the further shrinkage of the endoderm the two dorso- 

 lateral spaces fuse into one median dorsal space which subsequently 

 forms the cavity of the heart. Inasmuch as this space is wedged in 

 between the dorsal apices of a pair of somites, it corresponds both in 

 origin and position to the dorsal blood-vessel of Annelida. 



Meanwhile each somite has become divided into a dorsal portion 

 lying at the side of the median blood-space, and a ventral portion lying 

 in the base of the corresponding appendage. The walls of both 

 portions, but especially of the latter, give rise by proliferation to a 

 great mass of cells which fills up the appendage and clings to the 

 side of the gut. In this mass other blood-spaces make their 

 appearance. First appears a space in each appendage which 

 embraces the tip of the ventral division of the coelom and forms the 

 cavity of the leg in, the adult. This we may term the appendicular 

 sinus. Then comes a space nearer the mid ventral line, above the 

 spot where the nerve-cord is formed. The nervous sytem arises in the 

 typical annelidan manner as two ventral band-like thickenings of 

 the ectoderm which remain widely separated in the middle line, but 

 which meet one another in front of the mouth and behind the anus. 

 The brain arises as two thickenings of the prae-oral lobe where these 

 bands meet in front. 



The blood-space above each half of the nervous system forms the 

 lateral sinus of the body cavity of the adult, and it remains separated 

 from the appendicular sinus and also from the median ventral blood- 

 space by a strand of cells. Above it, at the sides of the gut and 

 external to the dorsal divisions of the coelom, two other spaces appear 

 at each side. 



The dorsal division of the coelom, in most of the segments of the 

 body, collapses and forms a flat plate of cells from which the side of 

 the heart and one half of the pericardial septum are formed. One 

 pair of the spaces which lay externally to these parts of the coelom, 

 meet above the plates of cells which result from the collapse of the 

 coelomic cavities and form the pericardium ; the other pair meet 

 above the gut and form the dorsal division of the general body-cavity. 

 This dorsal division of the haemocoele coalesces with the me.dian 

 ventral space and forms the general perivisceral cavity of the 

 adult. The ventral division of the cavity of the somite (som'^, Figs. 

 125 C and D) — i.e. the true coelom — persists as a thin-walled vesicle 

 from which a coiled tube, the excretory organ, the so-called 

 "nephridium," grows out and, fusing with the ectoderm on the inner 

 side of the leg, forms there a pore which is, the external opening of 

 the nephridium. Finally the nervous system separates from the ecto- 

 derm, forming two parallel nerve-cords, and between them and the 

 ectoderm, of which they .originally formed a part, a sub-neural sinus 

 is formed. 



The ectodermic thickenings from which the nerve-cords have 

 broken away, remain for a long time visible, and are termed by 

 Sedgwick ventral organs. They gradually approach each other in 



