404 Mr. J. D. Macdonald on a new form of Compound Tunicata. 



b—- 



This, however, and other particulars, will be better understood 

 by reference to the annexed diagrams, which are principally in- 

 tended to show the theoretical changes through which a simple 

 Ascidian plan must pass, to assume successively the characters 

 of a common Ascidian, a Boltenia, and a zooid of the genus now 

 under consideration, for which I shall adopt the provisional 

 name of Chondrostachys. 



A, simple plan of an Ascidian, with the ventral surface turned 

 downwards. The entering arrow 

 shows the branchial orifice with its 

 tentacula, and the emerging one the 

 cloacal outlet : a, the branchial sac ; 

 b, the mouth and oesophagus ; c, the 

 stomach j d^ the intestine and vent ; 

 e, the cloacal chamber. A vertical 

 line is drawn through the nervous ^' 

 ganglion (indicated by a black spot) 

 and the viscera, dividing the body 

 into an anterior and a posterior half; 

 and the deviation of this axis from 

 that of the plan is represented by a 



corresponding line in the other diagrams, in which also the 

 several parts marked in the plan will be readily recognized. 



The plan, in the position given, to some extent resembles a 

 zooid of Pyrosoma ; but if its ventral surface be turned upwards, 

 so that the body may rest principally on the posterior dorsal 

 region, the stomach will then hold a low position ; and the in- 

 testine, somewhat more lengthened, will naturally take the flex- 

 ures which it exhibits in the ordinary Ascidian (B). 



B 



Let us now suppose this latter form to be suspended from the 

 point 1, B ; the body will become elongated, i.e. both compressed 

 laterally and depressed in the direction of its vertical axis. The 

 position of the mouth and vent will be lowered, and the loop of 



