Messrs Forbes and Goodsir on Pelonaia. 33 



placed more longitudinally. The respiratory sac is longer, 

 the stomach is longer, and is not placed so much across the 

 body. The oesophagus runs down to the bottom of the sac 

 before it terminates. The rectum is very long, and of con- 

 siderable 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, carti- 

 laginous in appearance, 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 

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

 the Tunicata. They difix?r from the other Ascidice more parti- 

 cularly in being bilateral. The generative organs are sym- 

 metrical, and open one on each side of the anus, which is 

 directed towards the ventral surface of the animal, in a line with 

 the mouth and nervous 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 articulata. 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 ves- 

 sel exists) in the annulosa. It is interesting also to perceive 

 that, coexisting with this decided approach to the annular 

 type of form, we have the transverse plaits of the respiratory 

 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 Ascidice, and an approach to other 

 types of form, and more particularly to the Cirro-grade Echi- 

 nodermata, with certain of which Pelonaia has at least an 

 analogical relation, in the water filled body, and in the ex- 

 ternal form. 



Pelonaia, in fine, is one of those connecting genera so valu- 

 able as filling up gaps in the system, and supplying links in the 



VOL. XXXI. NO. LXI. JULY 1841. C 



