■ MOLLUSCOIDA : TUNICATA. 289 



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 



Where the atrial tunic is reflected over the sides of the pharynx, 

 the two enter into a more or less complete union, and the sur- 

 faces of contact become perforated by larger or smaller, more 

 or less numerous, apertures. Thus the cavity of the pharynx 

 acquires a free communication with that of the atrium ; and 

 as the margins of the pharjmgo-atrial apertures are fringed with 

 cilia working towards the interior of the body, a current is 

 produced, which sets in at the oral aperture and out by the 

 atrial opening, and may be readily observed in a living 

 Ascidian." — (Huxley.) 



As regards some points in the above description. Professor 

 Allman does not agree with Huxley, but believes, on the other 

 hand, "that the walls of the atrium simply surround the 

 branchial sac, without being reflected on its sides, and that the 

 branchial sac is therefore properly, within the cavity of the 

 atrium." 



In structure, the pharyngeal or "branchial" sac is composed 

 of a series of longitudinal and transverse bars, which cross 

 each other at right angles, and tlius give rise to a series of 

 quadrangular meshes, the margins of which are fringed with 

 vibratile cilia. These bars are hollow, and are really vessels 

 v/hich open on each side into two main longitudinal sinuses, 

 the so-called "branchial" or "thoracic" sinuses — one of which 

 is placed along the h^mal side of the pharynx, whilst the . 

 other runs along its neural aspect. The function of the entire 

 perforated pharynx is clearly respiratory. 



The Tunicata possess a distinct heart, consisting of a simple 

 muscular tube, which is open at both ends, and is not provided 



