MOLLUSCOIDA : TUNICATA. 267 



loosely attached to it, especially near the apertures. The 

 mouth is usually surrounded by a circlet of small, non-ciliated, 

 non-retractile tentacles, and opens into a large chamber (fig. 

 96, i, c\ which usually occupies the greater part of the cavity 

 of the mantle, and has its walls perforated by numerous aper- 

 tures. This is known variously as the " pharynx," the " respi- 

 ratory sac," or the " branchial sac." (It must be remembered 

 that the aperture here spoken of as the mouth can only be 

 looked upon in this light provided that the respiratory sac is 

 looked upon as the pharynx. By Professor Allman, whose 

 definition is given at the head of this chapter, this view is not 

 accepted, and consequently the internal or inferior opening of 

 the respiratory sac is regarded as the true mouth.) Inferiorly 

 the respiratory sac leads by a second aperture into an oeso- 

 phagus, which opens into a capacious stomach. From the 

 stomach an intestine is continued, generally with few flexures, 

 to the anal aperture, which does not communicate directly 

 with the exterior, but opens into the bottom of a second cham- 

 ber, which is called the " cloaca " (fig. 96, i, f). Superiorly 

 the cloaca communicates with the external medium, by means 

 of the second aperture in the test. 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, therefore, 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 communication with 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 ' 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 * en- 

 dostyle.' It then passes from the sides of the pharynx to the 

 body- walls, on which the right and left lamellae become con- 

 tinuous, 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 



