26 ARTHUR E. SHIPLEY. 



These authors describe aud figure in all stages the development 

 of the heart and the pericardium of this Ascidian from an out- 

 growth of the ventral wall of that part of the endoderm which 

 forms the pharynx, close to the end of the endostyle. This hollow 

 diverticulum becomes separated from the endoderm and lies as 

 a closed vesicle outside it. One half of the vesicle then in- 

 vaginates, so that a two-walled vesicle results, there being a space 

 left between the outer and inner wall. This space becomes the 

 cavity of the pericardium, whose wall is formed of the outer layer 

 of the double vesicle ; this cavity is derived from the cavity of the 

 endoderm. 



The inner wall of the vesicle forms the wall of the heart, and 

 the cavity of the heart is continuous with the primitive body- 

 cavity. The longitudinal opening from the heart into the body- 

 cavity persists for some time, until the free swimming larval stage; 

 eventually it closes in the middle but still leaves an anterior and 

 posterior opening through which the blood enters the heart from 

 the body-cavity and leaves it again each time that organ contracts. 



In Kleinenberg's' remarkable paper on the larva of Lopado- 

 rhyncus, he states that the segmentation cavity becomes the coelom 

 in this and in many other Annelids. The coelom is therefore in 

 these animals archi-coelic in nature, and we have seen that in some 

 Vertebrates the vascular system is of this nature. In the Nemertea 

 the spaces which may be perhaps considered to be both body- 

 cavity and vascular cavity are also archi-coelic. This group of 

 animals would therefore seem to have retained the most primitive 

 of all cavities — the segmentation cavity — as the only system of 

 spaces between the endoderm and ectoderm : whilst the primitive 

 segmentation cavity has differentiated in the higher animals, on 

 the one hand into body-cavity — Annelids, and on the other in 

 the cavities of the vascular system — Vertebrates. 



1 " Die Entstehung des Annelids aus der Larve von Lopadorhynous." Kleinen- 

 berg, Zeit. f, wis. Zoologie, Bd. 44, 1886. 



