404 COMPAEATIVE ANATOMY. 



forms an oesophagus, which has a funnel-like commencement in the 

 Copelata. A second, generally wider, section may be regarded as 

 stomach, and corresponds to a mid-gut. In Ascidians it is broken up 

 into numerous smaller chambers by a number of folds and sagittate 

 up-growths of the wall; in Copelata it has a csecal appendage. Caacal 

 structures are also found on the stomachs of many Salpse. The 

 section thus constituted is usually of considerable length in the 

 Ascidians, and returns on itself in the form of a loop, from one arm 

 of which the hind-gut proceeds. These two sections of the canal 

 ai*e quite short in the Copelata and also in the Cyclomyaria, where, 

 as in the Ascidians, they are but little differentiated from one another. 

 In many Ascidians, the single or double loop of the alimentary 

 canal has a position at the side of the respiratory chamber in the 

 outgrowth of the body-cavity which there surrounds the respiratory 

 sac ; others have the digestive tract confined entirely to the region 

 behind the respiratory chamber, the various forms of which appear 

 to determine this relation. 



The Salpse have their digestive tract, together with its appen- 

 dages, compacted into a mass (the nucleus). 



As accessory organs of the digestive tube we have, besides 

 the outgrowths already mentioned, other glandular tubes in all the 

 higher divisions. These tubes open into the sectiou of the canal 

 which serves as stomach. There can be no doubt that they furnish 

 a secretion adapted to the purposes of digestion. In form and 

 arrangement they exhibit many variations. Sometimes they form 

 net-like anastomoses. 



Chandelon, Th., Rech. sur une annexe du tube dig. des Tuniciers. Bull. Acad. 

 Belg. XXXIX. 



Vascular System. 

 § 313. 



In the arrangements of their circulatory organs, the two great 

 divisions of the Tunicata differ from one another. In the Copelata 

 a heart only is known, and that is absent from one genus. It con- 

 sists of a short sac with its ends attached to two cells, whilst its 

 delicate walls present two longitudinal slits placed on opposite sides 

 to one another. The circulation of the blood is provided for by 

 the pulsations of this pouch; in fact, without the existence of 

 vessels, a distinct direction is given to the movement of the blood 

 in the body-cavity which can be recognised. In the Acopa a vas- 

 cular system in connection with the heart is present, which in certain 

 parts has a lacunar character. It appears then that a portion of 

 the primitive body-cavity is adapted to tho purposes of a blood- 

 carrying system. 



In the Ascidians the elongated heart lies in the neighbourhood 

 of the digestive organs, and bending inwards at each end gives rise 



