Respiration in Invertebrate Animals. 4'5 



pharyngeal chamber by the water drawn in by the extra-branchial 

 siphon. Nature's machinery would then, indeed, wear the dis- 

 graceful impress of faultiness. 



In the Ascidians the branchiae completely line the walls of the 

 pallial chamber. In figure the chamber varies ; it is oblong in 

 some species, oval and rectangular in others. The branchial 

 membrane in Ascidia, Phallusia, &c. forms a plane unfolded sheet, 

 adapting itself to the cavity of the mantle ; in Cynthia, Boltenia, 

 &c. it is longitudinally plicated (tig. 5) and disposed in deep and 

 regular folds. The ultimate vessels (d) are arranged rectangu- 

 larly. The circumscribed ' stigmata ' (c) are parallelogrammic in 

 figure. These perforations lead from the pharyngeal into the 

 " thoracic " chamber of Milne-Edwards. Why it should be 

 called ' thoracic ' is difficult to understand. As already denned, 

 it is really the visceral, intra-branchial or cloacal cavity. The bran- 

 chial vessels in the Ascidians are arranged in two planes (tig. 4). 

 In Cynthia ampulla the meshes are very irregular and almost 

 inextricable, some of the minute vessels having apparently a 

 spiral arrangement. In Chelyosoma, Eschricht figures a similar 

 vermicular disposition of the branchial vessels. The branchial 

 membrane of Cynthia presents large longitudinal vessels. They 

 are crossed by others of equal size. Large meshes (d) are thus 

 formed. Smaller vessels (b) lying on a different plane form by 

 crossing smaller stigmata. In Ascidia and Chelyosoma, the angles- 

 of the meshes of the branchial membrane bear papilla (c) more or 

 less prominent. In Cynthia they do not exist. These papillose 

 processes are hollow recesses. They are by-receptacles for the 

 nutritive fluid. In size the branchial vessels vary in different 

 genera. In Cynthia they are large, in Ascidia they are minute, 

 in Cystingia they are indistinct. The branchial plicae converge at 

 the mouth whenever they exist. 



By Carus and Van Beneden a lateral opening in the respi- 

 ratory cavity has been indicated, by which the water passes di- 

 rectly from the branchial sac into the cloaca (fig. 1, o; fig. 3, e). 

 This aperture corresponds with the open fissure which in many 

 species of Acephalans exists between the attached border of the 

 branchiae and the base of the foot. It is a safety-valve, as will 

 be hereafter explained. 



In Clavellinidse, Botryllidae, in the genera Pyrosoma, Pelonaia 

 and Salpa, such is the structure of the branchiae, that the water 

 readily traverses the respiratory stigmata, and passes from the 

 extra-branchial into the intra-branchial chambers. 



In all genera the branchial membrane is attached by means 

 of threads and vessels externally to the mantle. 



The branchiae in the Clavellinida? exist in form of a band 

 stretching across the cavity of the mantle, and dividing the pha- 



