370 TUNICATA. 



be described, in a thin continuous stream, sometimes projected to 

 a distance of many inches. 



(403.) Respiration is effected in an apparatus of very peculiar 

 contrivance ; to the examination of which we must now request the 

 attention of the student. A considerable portion of the interior 

 of the body is occupied by a circumscribed cavity, that opens 

 externally by the orifice h ; into this bag a bristle has been intro- 

 duced, in the dissection represented in the figure (Jig. 174) : its 

 walls are seen to be composed of a thin but very vascular mem- 

 brane (dj d, d), that has been partially turned back, so as to dis- 

 play the interior of the respiratory sac. The membrane (Jig. 174, 

 d, d, d ; Jig. 175, e), when examined 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 dis- 

 covered to be densely studded with vibratile cilia, whose rapid 

 action constantly diffuses fresh supplies of water over the whole 

 vascular membrane. The respiratory cavity has but one orifice 

 for the admission of water (Jig. 175, a) ; and this is guarded by 

 a fringe of delicate and highly sensible tentacula (fig. 175, b) ; 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. 



(404.) The heart itself presents the simplest possible form ; be- 

 ing generally a delicate elongated contractile tube, receiving at one 

 extremity 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 circulating fluid, and disperses it through the system. 



The heart, above described, is extremely thin and transparent, 

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

 the other viscera. 



(405.) When we consider the fixed and immoveable condi- 

 tion of an Ascidian, and its absolute deprivation of all prehen- 

 sile instruments adapted to seize prey, it is by no means evident, 



