On Subdivision of iJte Median Fin in a Dipnoan Fish. 241 



been already stated, tlie genital organs, and here also the 

 comparison with Pcripatits may be instituted. We find a 

 similar agreement to that which existed in the other regions 

 compared, and we may, with a considerable degree of proba- 

 bility, again homologize corresponding ])arts. The genital 

 organs of Pala^monetes must then be regarded as the represen- 

 tatives, in this region, of the coelom. 



If the homologies here suggested are valid, the body-cavity 

 relations of the Crustaceans under consideration may be stated 

 briefly thus : — both enterocoele (true coelom) and pseudocode 

 are present, the enterocoele consisting of the dorsal sac, the 

 green gland, and shell-gland, or the end sacs of these organs, 

 together with the genital organs and their ducts, whilst the 

 ])ericardial sejjtum may be regarded as equivalent to portions 

 of the walls of another part of the same structure. 



The pseudocoele consists of the heart and arteries, the peri- 

 cardial cavity, the central cavity of the thorax, with the 

 lateral cavities and the cavities of the limbs, and the various 

 sinuses of the abdomen. The pseudocoele is filled with blood, 

 and hence can be termed a htemoccBle. 



I hope shortly to publish a more detailed account, with 

 figures, of the relations described in this communication. 



XLI. — Note on a Case of Subdivision of the Median Fin in a 

 Dipnoan Fish. By A. Smith Woodward, F.L.S. 



Paleontology has demonstrated that in the Crossoptery- 

 gian Teleostomatous fishes the process of subdivision of the 

 median fin is usually different from that observed in the more 

 specialized Actinopterygians. Whereas in the latter case 

 portions of the fin-membrane disappear and the endoskeletal 

 supports afterwards become atrophied, in the former case it is 

 the almost universal rule that the supports of the dorsal and 

 anal region become concentrated in clusters, and thus induce 

 subdivision of the primitively continuous fin. The proximal 

 fin-supports (axonosts) in each of these clusters ordinarily 

 fuse into a single club-shaped element, and the distal supports 

 (baseosts) are often more or less reduced in number by the 

 same process*. 



• Sniitb "Woodward, ' Catalogue of Fossil Fishes in the liritisli Mu- 

 seum,' part ii. (1891); aud "The Evolution of Fins," Natural Siieijce, 

 vol. i. pp. 28-35 (1892). 



