242 MANUAL OP ZOOLOGY. 



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 pharyngo- 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 thus 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, 

 which open on each side into two main, longitudinal sinuses, 

 the so-called 'branchial,' or 'thoracic' sinuses one of which 

 is placed along the haemal 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 

 with valves. In consequence of this, the circulation, in the 

 majority of Tunicaries, is periodically reversed, the blood 

 being propelled in one direction for a certain number of con- 

 tractions, and being then driven for a like period in an oppo- 

 site direction ; ' so that the two ends of the heart are 

 alternately arterial and venous.' 



The nervous system consists of a single ganglion placed on 

 one side of the oral aperture, between it and the anus, in all 

 known Tunicata, except in the aberrant form Appendicularia. 



The only organs of sense are pigment-spots, or ocelli, placed 

 between the oral tentacles, and an auditory capsule, sometimes 

 containing an otolith. These organs, however, do not appear 

 to be constantly present. 



