Respiration in Invertebrate Animals. 251 



cumference of the disc. The system of the gastro-vascular 

 canals (fig. 5, c, d; fig. 6, c, d; fig. 7, c, d) in the Discophorte, 

 forming a horizontal plane, rest in immediate contact with 

 the inferior surface of the disc — that is, the whole substance 

 of the disc intervenes between them and the upper surface: 

 the under surface of the disc externally, in every species, is 

 ciliated ; the superior is not so. The stomach and the canals 

 (c, d) to their remotest terminations are cihated internally. 

 This fact distinguishes these canals fundamentally from blood 

 vessels ; they are filled with a fluid which is imperfectly vitalized, 

 a chylaqueous compound ; it is replete with floating organized 

 corpuscles. The flux and reflux motions of this fluid are excited, 

 partly by ciha, and partly by the rhythmic contractions of the 

 disc. Respiration is accomplished in two modes; partly by the 

 interchange of gases on the under surface between the contents 

 of the canals and the surrounding element, and partly by the 

 air suspended in the external fluid, which is admitted through 

 the mouth and stomach into the gastro-vascular channels directly 

 from without. 



The basis and bulk of this fluid is composed of salt water, but 

 qualified by the impresses of the zoochemical influence to sustain 

 the life of albumen, fibrine, and to evolve definitively organized 

 floating corpuscles. The refuse portions of this fluid are rejected 

 per OS ; there is no anal outlet. The cells of the solid structures 

 of the Acaleph are filled with a semifluid hyaline jelly ; it is the 

 chylaqueous fluid in its highest grade of organization. In the 

 Medusa, it is to the chylaqueous fluid, what the contents of the 

 " protean " cells of the gelatinous cortex are to the currents of 

 the circumambient element, traversing the passages in the 

 sponge : thus, in brief, is conveyed a description of the machinery 

 of the respiratory process in the Acaleph ; from it the nutritive 

 processes cannot be distinguished. The Ciliograde family departs 

 from the type of the former in one particular ; there exists here 

 a second orifice to the digestive system (fig. 7, b). The fact alters 

 not the principle of the mechanism, according to which the fluids 

 are aerated. ^The gastro-vascular canals arise from the fundus of 

 the stomach, attain the sm*face, and pass in meridional series 

 (fig. 7,c,d) from one pole of the body to the opposite, lying im- 

 mediately underneath the external epidermis. Their courses are 

 followed externally by rows of motive cilia, or vibratoiy fringes : 

 all the canals peripherally terminate csecally ; they are furnished 

 on their internal surfaces with cilia. The genera Cydippe, Cestrum 

 and Callianira are illustrative. 



In the Cirrhigrade Acalephs, the second orifice of the alimentaiy 

 apparatus disappears. The canals, filled with the chylaqueous 

 fluid, radiate, while they multiply in the direction of the cir- 



