MOLLUSCA: TUNICATA. 383 



more or less complete union, and the surfaces of contact be- 

 come 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 to- 

 wards 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 " 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 sac, whilst the other runs along 

 its neural aspect. The function of the entire perforated sac 

 is clearly respiratory. 



The Tunicata mostly possess a distinct heart, consisting of 

 a simple muscular tube, which is open at both ends, and is 

 not provided with valves. The circulation is attended with 

 the remarkable peculiarity of being periodically reversed, the 

 blood being propelled in one direction for a certain number 

 of contractions, and being then driven for a like period in an 

 opposite 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, usually 

 placed between the oral tentacles, and an auditory capsule, 

 sometimes containing an otolith. These organs, however, are 

 not constantly present. 



With the exception of Doliolum and Appendicularia, all the 

 Tunicata are hermaphrodite. Mr Saville Kent, however, is of 

 opinion that Appendicularia is the free reproductive zooid of 

 an Ascidian, bearing to the adult the same relation that the 

 Medusae do to the Hydroid colony. The reproductive organs 

 are situated in the fold of the intestine, and their efferent duct 



