464 



The Vortex Ring Theory of Gases, 



[June 18, 



between the setae of a given segment are certain peculiar bodies 

 having the appearance of unicellular glands, and surrounded by an 

 inward prolongation of the cuticle. Quite similar structures have 

 been met with by Vejdovsky in Anachceta* and in this worm they 

 replace the setce ; there is therefore evidently some reason for supposing 

 that Urochoeta has descended from ancestors in which there was a 

 complete row of setae round each segment. 



M. Perrier is of opinion that the quadriserial arrangement referable 

 to the parapodia of the Polychaetous worm is the primitive arrange- 

 ment, and that the condition found in Perichceta may have been 

 arrived at, first, by a separation of the setae of each pair, and 

 secondly, by the development of the secondary setae (" soies de 

 r emplacement"), which might get to be placed alongside of the 

 primary setae and form new primary setae. 



It appears to me just as legitimate to suppose a. process of reduction 

 from a more generalised condition, and there is strong evidence that 

 this has taken place in Urochoeta, and that the resemblance between the 

 more common disposition of the setae and the parapodia of the 

 Polychaetous worm is due to adaptive rather than to any genetic 

 causes. 



Such an hypothesis would fit in very well with the new facts 

 recorded in the present paper, for I have been able to show that there 

 is no connexion between the pair of setaef and the nephridium, as 

 between the parapodium and nephridium in the Polychaeta, and that 

 therefore, in so far as these facts have weight, the dorsal and ventral 

 pairs of setae do not correspond to the parapodia. 



XV. " The Vortex Ring Theory of Gases. On the Law of the 

 Distribution of Energy among the Molecules." By Pro- 

 fessor J. J. Thomson, B.A., F.R.S., Fellow and Lecturer 

 of Trinity College, Cambridge. Received June 4, 1885. 



[Publication deferred.] 



* " Monographic der Enchytraeiden," p. 21. 



f It is worth recording the fact that in a species of Acanthodrilus, also from 

 New Zealand, where the setae are paired the nephridial orifice is not placed indefi- 

 nitely in front of the pair, but has a distinct relation to one of the two setae. 



