CHORDATA 443 
of attachment of the hypopharyngeal groove; the other arm 
is continued into a cardio-visceral vessel, which breaks up into 
many splits or sinuses amongst the viscera. From these splits 
in the tissues of the various organs the blood passes into a 
Fic, 258.—Diagram of 
circulation in a simple 
Ascidian. Herdman. 
br. Mouth. z. Intestine. 
at. Atrial aperture. bc. Branchiocardiac or ventral vessel. 
a. Anus. cv. Cardiovisceral vessel. 
brs. Branchial sac. vb. Viscerobranchial or dorsal vessel. 
oe. Oesophagus. vt. Vessels to test. 
h. Heart. 
viscero-branchial vessel situated at the base of the dorsal 
lamina. From this the blood passes into the vessels in the 
lateral bars of the branchial sac which carry the blood down 
to the ventral vessel, and it thus returns to the heart. A 
peculiarity of the circulation in Ascidians is that its course 
is from time to time reversed, after contracting for a certain 
number of times in one direction the heart stops and then 
recommences in the other, thus reversing the course of the 
circulation. 
The vessels arising from each end of the heart give off 
branches into the test; these subdivide, and their smaller 
branches usually end in spherical cavities (Fig. 252). They 
probably serve to nourish the test and keep it alive, and in 
some species they may possibly play some part in respiration. 
The blood contains amoeboid and spherical corpuscles. 
The cavity of the heart and of the blood sinuses con- 
nected with it is directly derived from the blastocoel or 
segmentation cavity of the embryo; this is an interesting fact, 
taken in conjunction with the similar origin of the space of 
the heart in the lamprey, and possibly in other Vertebrates. 
The nature of the space in which the stomach, ovary, and 
pericardium lie, is at present a matter of some uncertainty ; 
it contains a corpusculated fluid. 
