Wall of the Body-cavity of Vertebrates. 



245 



very early age and they are so lost in the Cyclostomata. It is, 

 therefore, not clear whether that intermediate group of Selachians 

 which has both nephrostomes and abdominal pores is on the way to 

 lose the former and depend on the latter or vice versa. 



With regard to the homology of the pores in the different groups 

 of fishes and in reptiles, the answer will depend on the proof or dis- 

 proof of Balfour's homology. If the pores are not segmental tubes, 

 they are simply perforations of the abdominal wall in consequenc e 

 of gradual thinning down in the cloacal region. Should this be so, 

 it is evident that this process may have taken place independently 

 many times over in the phylogeny of the different groups, and there 

 would be great difficulty in establishing the homology between any 

 two groups of Vertebrates in respect to the pores. 



Let us glance for a moment at the role played by the body-cavity 

 in the series of changes from its original condition (i) as a part of 

 the excretory system, (ii) as a part of the reproductive system receiv- 

 ing the genital products as they are set free, and (iii) as a part of 

 the lymphatic system, receiving the transudations of the visceral 

 and abdominal walls. From this condition it has been specialised, 

 losing first the sexual part of its duties, when the ovaries and testes 

 became more or less continuous with their ducts. It would after- 

 wards, as in the Anura, become less an excretory organ and more a 

 lymph reservoir, and similarly in the higher reptiles and mammals 

 it becomes more and more specialised as a part of the system of lym- 

 phatics. 



I must record my gratitude to Mr. Boulenger for facilities granted 

 in examining specimens under his charge at the British Museum, to 

 Mr. S. F. Harmer, Superintendent of the Museum of Comparative 

 Anatomy, Cambridge, for the use of specimens in the collection, and 

 lastly, I owe much gratitude to Dr. Hans Gadow, for the encourage- 

 ment and suggestive assistance he has always been ready to give me. 



Summary. 



1. There is a reciprocal and compensating correlation in the adult 

 Elasmobranchii, Ganoidei, Dipnoi, some Teleostei, Amphibia, certain 

 Chelonia and Crocodilia in the distribution of nephrostomes and of 

 abdominal pores. In some Selachians only are both present. In 

 the majority of the Elasmobranchii, and in all the other groups, the 

 presence of one set of organs excludes the presence of the other. In 

 the higher Teleostei, in Hatteria, some Crocodiles and Chelonians, 

 both have been lost. Anura hold an intermediate position in so far 

 as the nephrostomes are present, but are no longer connected with 

 the renal system, and the body-cavity communicates with the circu- 

 latory system through two channels, (I) through the nephrostomes, 



