46 PELONAIAD^. 



Anatomy of P. corrugata. 



" The structure of this species differs very little from that of P. 

 ylahra. The animal being elongated, the organs are placed more 

 longitudinally. The respiratory sac is longer ; the stomach is 

 longer, and is not placed so much across the body. The oesopha- 

 gus runs down to the bottom of the sac before it terminates. The 

 rectum is very long, and of considerable width, but just before it 

 terminates in the anus it becomes very much contracted. The 

 mantle exhibits no ridge or shelf below the anal orifice, but its 

 longitudinal fibres are very strong, and form a thick bundle at 

 their origin round the respiratory opening. The test, instead 

 of being thin and diaphanous like parchment, as in P. glabra, is 

 thick and cartilaginous, coloured brown, and transversely wrinkled 

 externally. 



" From the details of structure which we have now given, it is 

 evident that the Pelonaice are Ascidice. Their anatomy is impor- 

 tant, as it explains the nature of the parts and organs in the 

 Tunicata. They differ from the other A scldice more particularly 

 in being bi-lateral. The generative organs are symmetrical, and 

 open one on each side of the anus, which is directed toward the 

 ventral surface of the animal, in a line with the mouth and ner- 

 vous ganglion. The latter is thus proved to be an abdominal or 

 sub-oesophageal ganglion, corresponding to, or forming one of the 

 chains of ganglia on the abdominal surface of the Artmdata. In 

 the same manner, the branchial artery or heart is proved to be the 

 pulsating dorsal vessel, and the branchial vein the abdominal 

 vessel (when that vessel exists), in the Anmdosa. It is interest- 

 ing also to perceive, that, co-existing with this decided approach 

 to the annular type of form, we have the transverse plaits of the re- 

 spiratory sac corresponding to the rings of an articulated animal. 

 The disappearance of a separate test is also a departure from the 

 plan of formation in the Ascidiw, and an approach to other types 

 of form, and more particularly to the cirrhograde Echinodermata, 

 with certain of which Pdonaia has at least an analogical relation, 

 in the water-filled body and in the external form. 



" Pelonaia, in fine, is one of those connecting genera so valuable 

 as filling up gaps in the system, and supplying links in tlic chain 

 of structures which runs through the series of organised bodies." 



