CHORDA TA 



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 



br 



br. Mouth. 



at. Atrial aperture. 



It. Anus. 

 hrs. Branchial sac. 



oe. Oesophagus. 



h. Heart. 



Fig. 2.58.— Diagram of 

 circulation in a simple 

 Ascidian. Herdman. 



i. Intestine. 

 Ic. Branchiocardiac or ventral vessel. 

 cv. Cardiovisceral vessel. 

 vh. Viscerobranchial or dorsal vessel. 

 vt. Vessels to test. 



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. 



