Z BRITISH TUNICATA. 



relative position of these organs in Clavelina and the 

 compound Ascidians. By this latter character dona 

 seems to be related to Pelonaia, and also by the much 

 elongated form of the branchial sac, and the position 

 and proportions of the alimentary canal. It differs 

 from both these genera [Clavelina and Pelonaia] by 

 the oral lamina being replaced by a series of filaments. 



The test of Ciona is not only characterized by its 

 softness and contractility, but also by the facility with 

 which it separates into layers, and by the fewness and 

 indefiniteness of the vascular ramifications in its sub- 

 stance. 



The mantle is remarkable for the arrangement of 

 the muscular fibres, which are nearly as numerous on 

 the one side of it as 011 the other. They are accumu- 

 lated into ten stout, widely-separated, longitudinal 

 bundles, which, originating at the base of the mantle, 

 five on each side, extend upwards and pass into the 

 walls of the respiratory tubes ; three terminating in 

 the inhalent, two in the exhalent tube. Delicate 

 transverse fibres can also be traced the whole length 

 of the mantle, particularly 011 the left side. The ocelli 

 at the entrance of the respiratory tubes are very con- 

 spicuous. 



The branchial sac is much elongated, and is com- 

 posed of a simple rectangular reticulation of vessels ; 

 the primary or transverse channels being large and 

 regularly disposed, the secondary minute and longi- 

 tudinal. The longitudinal bars are stout, and have at 

 the intersections very large, curved, tubercular pro- 

 cesses. 



The heart is also much modified. It lies in a fold 

 of the lining membrane within what may be called the 

 abdominal chamber, towards the dorsal margin, between 

 the bottom of the branchial sac and the anterior or 

 upper border of the stomach. It is in the form of a 

 long tube, doubled upon itself, and is enclosed within 

 a kind of pericardium, formed apparently by the lining 

 (suspending) membrane. The looped or double portion 



