HEART AND CIRCULATORY SYSTEM. 481 



with a microscope, is found to be covered with a magnificent network of 

 blood-vessels, formed by innumerable canals uniting with each other at 

 right angles ; and moreover, when seen in a living state, its surface is 

 discovered to be densely studded with vibratile cilia, whose rapid action 

 constantly diffuses fresh supplies of water over the whole vascular mem- 

 brane. The respiratory cavity has but one orifice for the admission of 

 water (fig. 244, a), and this is guarded by a fringe of delicate and 

 highly sensitive tentacula (fig. 244, 6) ; so that the water, as it is drawn 

 into the body, having necessarily to pass these tactile organs, any foreign 

 substances which it might contain of a prejudicial character are at once 

 detected and denied admission. All the vascular ramifications spread 

 over the lining membrane of the branchial chamber are connected with 

 two sets of large vessels ; one of which, receiving the blood from the 

 body, disperses it over the spacious respiratory surface ; while the other, 

 collecting it after it has undergone exposure to the respired medium, 

 conveys it in a pure state to the heart. 



(1263.) The heart itself presents the simplest possible form, being 

 generally a delicate elongated contractile tube, receiving at one extre- 

 mity the blood derived from the numerous vessels that ramify over the 

 interior of the branchial sac, whilst at the opposite end it becomes 

 gradually attenuated into the aorta, through which it impels the circu- 

 lating fluid and disperses it through the system. 



(1264.) The heart, above described, is extremely thin and trans- 

 parent, and is lodged in a distinct pericardium, which separates it from 

 the other viscera. 



(1265.) Notwithstanding this apparently simple arrangement of the 

 vascular system in the Ascidians, the nature of the circulation of the 

 blood, throughout the class, is extremely curious, the action of the heart 

 being completely reversed at brief intervals, and the course of the blood 

 entirely changed a phenomenon which is easily witnessed in any of 

 the smaller and more transparent species, when placed under the mi- 

 croscope. The contractions of the heart succeed each other with regu- 

 larity ; but they are sluggish, not extending at once through the whole 

 organ. The systole commences at one extremity, and is propagated by 

 an undulatory movement towards the opposite end by a sort of peri- 

 staltic action. For some time the contractions succeed each other with 

 rapidity, passing on in the same course, when they suddenly cease, and, 

 after a pause, recommence from the other end of the viscus. The blood, 

 thus impelled alternately from behind forwards, and then in the contrary 

 direction, ascends towards the branchial apparatus ; nevertheless it docs 

 not appear to be conducted there by closed vessels, but seems to be 

 diffused between the inner tunic of the abdomen and the viscera, where 

 it flows in currents that vary in their direction as the movements of 

 the animal, or any other mechanical causes, affect their passage. The 

 chief portion of the blood, however, ascends by the dorsal or the ventral 



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