158 Mr. J. Alder on the British Tunicata. 



placed the viscera ; the smaller branchial meshes, moreover, are 

 rectilinear. In the present species the animal is attached by the 

 base, near which the viscera are placed, but on the left side. 

 There is another peculiarity attending this species, namely, that 

 the flexure of the iutestine is in a contrary direction to what is 

 usual in the genus. The intestine, after leaving the stomach, 

 usually rises upwards on the right side of the branchial sac, and, 

 afterwards bending downwards, forms a sigmoid curve, rising 

 again towards the anal orifice. In this species the intestine, 

 after leaving the stomach, bends immediately downwards on the 

 left side, and, running along the base, rises towards the anal 

 orifice*. It will be seen from these details that A. parallelo- 

 gramma is a true sinistral species. But the chief peculiarity of 

 this interesting Ascidian is in the meshes of the branchial sac, 

 which are beautifully convoluted in a spiral direction on a fiat sur- 

 face (PL VII. figs. 1 & 2). From the centre of each spiral, smaller 

 bands radiate to the circumference, which serve to hold the 

 spiral vessels in their places. This delicate and complicated 

 system of vessels is traversed by larger longitudinal vessels, 

 which are on a different plane, and internal in position to the 

 other portions of the sac, to which they are united by broad 

 transverse vessels, that rise to them in a kind of loop, the apex 

 of which forms the papilla. The spiral vessels are thus set in a 

 square frame, the only part that is visible until a high magnifier 

 is applied. This is perhaps the reason why this beautiful struc- 

 ture has hitherto escaped observation, as the species is pretty 

 generally diffused, and from its beauty usually attracts attention. 

 This species also differs from the other Ascidice in having the 

 anal tube much the longer — a character ODly to be observed in 

 the living animal. A variety of it is the Ascidia virginea of 

 Forbes, but not of Muller. 



The Phallusia Turcica of Savigny is a sinistral species, having 

 a flexure of the intestine in the same direction as this; but the 

 branchial sac, though peculiar, has the meshes rectilinear, and 

 not convoluted. 



Genus Molgula, Forbes. 



The Molgula of Forbes was founded on external characters 

 only, the principal one being the number of segments in the 

 apertures — a character which has been taken by common con- 

 sent for generic distinction in this tribe, and, though of little 

 physiological importance, generally carries with it others of 

 greater functional value, so that the genera founded upon it 

 have usually been found to be natural. This has fortunately 

 been the case with the present genus, the internal characters 



* Both flexures are dorsal, but in different directions. 



