220 Mr A. E. Shipley, On the Vascular System. [May 7, 
kidney in Bdellostoma Forsteri. He finds running through the 
substance of this organ a number of fine tubules, lied with 
columnar cells and anastomosing with one another. These tubules 
open on the one hand into the pericardium and on the other into 
a central duct. In this duct lies a clot which is exactly similar 
to the blood clots found in the surrounding blood vessels. Further, 
in some cases capillaries were seen to enter this duct. There 
seems to be no reason to doubt that in this animal we have a part 
of the vascular system in communication with a part of the body- 
cavity through the tubules of the head kidney. 
That there is a very primitive connection between these 
systems, is further supported by the remarkable observations of 
Seeliger*, and Van Beneden and Julin® in the development of the 
heart of Clavellina. 
These authors describe and 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 
1 “Die Hntwicklungsgeschichte der Socialen Ascidien,’ Oswald Seeliger. 
Jenaische Zettsch. fiir Naturwissenschaft, 1885. 
2 “Recherches sur la Morphologie des Tuniciers,” Van Beneden and Julin. 
Gand, 1886. 
3 “ Die Entstehung des Annelids aus der Larve von Lopadorhyncus.” Kleinen- 
berg, Zeit. f. Wis. Zoologie, Bd. 44, 1886, 
