MOLLUSCOIDA I TUNICATA. 309 



and the surfaces 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 

 Allrnan 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 alter- 

 nately 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, some- 

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

 appear to be 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 



