440 PELONAIA, A NEW GENUS OF TUNICATED MOLLUSKS. 



dorsal vessel, and the branchial vein the abdominal vessel 

 (when that vessel exists) in the annulosa. It is interesting 

 also to perceive that, co-existing 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 de- 

 parture from the plan of formation in the Aseidice, and an 

 approach to other types of form, and more particularly to the 

 Cirrograde Ecliinodcrmata, with certain of which Pclonaia 

 has at least an analogical relation, in the water-filled body, 

 and in the external form. 



Pclonaia, in fine, is one of those connecting genera so 

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

 in the chain of structures, which runs through the series of 

 organised bodies. 



