50 THE VOYAGE OF II.M.S. CHALLENGER. 



considers that this body may be regarded as the representative of the glandular portion 

 of the organ of Bojanus. 



In some of the Molgulidse (the genus Lithonephrya of Giard) this renal organ is 

 always found occupied by a single large concretion, which almost entirely fills its cavity. 



In many of the Ascidiidse we find another kind of renal apparatus. A large number 

 of huge clear-walled spherical vesicles are aggregated together in a mass lying around the 

 intestine, and in the thickness of the mantle (fig. 9, r.o., p. 40), while a number of small 

 blood-vessels and sinuses ramify through the mass. As in the case of the Molgulidse, there 

 is no excretory duct, and the elimination products are stored up in the thin-walled vesicles 

 in the form of spherical concretions concentrically laminated, and sometimes having shells 

 of different tints of yellow and brown, and are usually covered by a thickish coating of a 

 pure chalky white. These concretions contain a considerable amount of uric acid. 



Finally, as has been stated above, E. van Beneden and Julin have shown strong 

 grounds for the belief that the neural gland is a renal organ, especially devoted to 

 the nerve ganglion. If this be the case, then this gland differs totally from the other 

 renal organs in the Tunicata, inasmuch as it possesses a duct by which the eliminated 

 substances are excreted either into the branchial sac or the peribronchial cavity. 



Blood- Vascular System. 



The heart in the Tunicata is a simple sac or fusiform tube, from the two ends of 

 which vessels are given off. It is enclosed in a delicate membranous sac, the pericardium, 

 formed, according to Hancock, of a fold of the lining membrane of the peribronchial 

 cavity, to which it is attached along one side from end to end. 



In all Tunicates in which the matter has been investigated, the periodic reversal of 

 the blood current, first noticed by Kuhl and van Hasselt in the case of Salpa has been 

 found. The heart, after contracting for a certain time in one direction, stops, and then 

 the waves of contraction begin to pass along it in the opposite direction, consecraently 

 the blood is propelled from the dorsal and ventral ends of the heart alternately. In 

 the young Ascidia virginea there are usually from 35 to 40 contractions in one direction 

 at a time, and this takes from 1 min. 30 sec. to 2 min., whde the pause between the two 

 series of contractions is usually 7 or 8 seconds in duration, and thus equals three ordinary 

 contractions. 



The heart varies considerably in position in different forms. In the Molgulidse it is 

 placed on the right side, and adheres to the inner surface of the mantle immediately 

 anterior to the renal organ. 



In the Cynthiidse, as a general rule, it lies in close relation to the stomach or the first 

 portion of the intestine, but in the curious Cideolus, it is situated on the right side of 

 the body, close to the endostyle, and attached to the inner surface of the mantle. 



