382 MANUAL OF ZOOLOGY. 



the greater part of the cavity of the mantle, and has its walls 

 perforated by numerous apertures. This is known variously 

 as the "pharynx," the "respiratory sac," or the "branchial 

 sac." The last of these names is the best, as it is not certain 

 that the perforated respiratory sac is really the homologue of 

 the phaiynx. If this should be its real nature, then the bran- 

 chial opening in the test is truly the month ; but good authori- 

 ties regard the branchial sac as wholly unconnected with the 

 alimentary canal. Inferiorly the respiratory sac leads, by a 

 second aperture (fig. 200, A, o), into an oesophagus, which 

 conducts into a capacious stomach (g). If the branchial sac 

 be regarded as not representing a dilated pharynx, then its 

 lower aperture is the true mouth. 



From the stomach an intestine is continued, generally with 

 few flexures, to the anal aperture (an\ which does not com- 

 municate directly with the exterior, but opens into the bottom 

 of a second chamber, which is called the " cloaca," or " atrium " 

 (c). Superiorly the cloaca communicates with the external 

 medium, by means of the second aperture in the test (a). The 

 first bend of the intestine is such that, if continued, it would 

 bring the anus on the opposite side of the mouth to that on 

 which the nervous ganglion is situated. The intestine, there- 

 fore, is said to have a "haemal flexure;" whereas the flexure in 

 the case of the Polyzoa is "neural." The intestine, however, 

 in the Tunicata does not preserve this primary haemal flexure, 

 but is again bent to the neural side of the body, the nervous 

 ganglion coming finally to be situated between the mouth and 

 the rectum. As just stated, the anus is not in direct com- 

 munication wtth the exterior, but opens into a large cavity, 

 called the " cloaca," or " atrial chamber," which, in turn, opens 

 externally by the second aperture of the animal. This cloaca 

 is a large sac lined by a membrane which " is reflected like a 

 serous sac on the viscera, and constitutes the ' third tunic,' or 

 1 peritoneum.' " From the cloaca " it is reflected over both 

 sides of the pharynx" (respiratory sac), "extending towards 

 its dorsal part very nearly as far as that structure which has 

 been termed the ' endostyle.' It then passes from the sides of 

 the pharynx to the body-walls, on which the right and left 

 lamellae become continuous, so as to form the lining of the 

 chamber into which the second aperture leads, or the ' atrial 

 chamber.' Posteriorly, or at the opposite end of the atrial 

 chamber to its aperture, its lining membrane (the 'atrial 

 tunic ') is reflected to a greater or less extent over the intestine 

 and circulatory organs. . . . Where the ' atrial tunic ' is 

 reflected over the sides of the pharynx, the two enter into a 



