120 Dr. A. Krohn on the genus Doliolum and its species. 



neighbourhood of Salpa. The species was called D. denticulatum. 

 I am not aware whether this animal has been since examined by 

 other naturalists, although it occurs not unfrequently in the 

 MediteiTanean. 



Once, on a previous occasion, 1 found it at Messina ; but in the 

 course of last spring I took it frequently at Naples, and per- 

 suaded myself that not only in point of structure (not very per- 

 fectly made out by Quoy and Gaimard), but also in development, 

 it is decidedly an Ascidian. It is a free swimming Ascidian, 

 which in many respects closely resembles the SalpcB, and so far 

 forms an interesting transition between the two orders of the 

 Tunicata. The genus, however, is not limited to this one species, 

 as 1 discovered three other kinds at the same place and time. 



Before proceeding to describe the different species, it will be 

 desirable to consider what they all have in common. 



The genus Doliolum is mainly characterized by the circum- 

 stance, that the body of the animal (as the name indeed indi- 

 cates) resembles a cask open at each end. The anterior some- 

 what broader end is prolonged into a very short, often hardly 

 perceptible tube, which answers to the ingestive or respiratory 

 siphon of other Ascidians, and, like this, has its lip divided into 

 a number of segments. These lobes, generally about ten in 

 number, are somewhat pointed. At the opposite end, whose aper- 

 ture represents the cloacal aperture of other Ascidians, the body 

 becomes gradually narrower*. 



The mantle is relatively very thin, and contains scattered gra- 

 nules in its substance. The second layer of the body [Leibes- 

 schicht) is, as in all Tunicata, that which supports the nerves and 

 muscles. 



The nervous centre consists of a round ganglion placed in the 

 middle of the dorsal surface ; from it three antei-ior and as many 

 posterior branches proceed. Two of these pass divergingly to the 

 sides of the body, the third runs along the median line. The 

 muscular apparatus closely agrees with that of Salpa in its ar- 

 rangement. It consists, according to the species, of either eight 

 or nine flat bands, which, like hoops, encircle the body at tole- 

 rably regular intervals, and so give it a still stronger resemblance 

 to a little cask. 



The anterior band surrounds the base of the respiratory siphon, 

 the posterior immediately encircles the margin of the posterior 

 aperture. Both are less strongly marked than the other bands. 



With regard to the internal organs, the respiratory apparatus 



* The two apertures are diametrically opposite likewise in the Pyroso- 

 mata ; and here also the cloacal aperture, which opens into the cavity of 

 the common mass, has a smooth edge (see Savigny, Mem. sur les Anim. 

 sans Vcrtebres). 



1 



