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Notes on the Structure and Mode of Action of the 



"Oval" in the Pollack (Gad us pollachius) and Mullet 



(Mugil chelo). 



By 

 W. N. F. Woodland, D.Sc, 



Professor of Biology in the Midr Central College, Allahabad, India. 



AVith Seven Figures in the Text. 



During the summer of 1911 I conducted at the Plymouth Biological 

 Station a series of experiments on the living active gas-gland 

 associated with the bladder of certain marine fish, the results of 

 which are recorded in a paper published in the Anatomisclicr Anzeiger 

 for 1911 (Bd. XL, p. 225). Whilst so employed I incidentally made 

 some observations on the structure and mode of action of the " oval " 

 in fishes, and since my conclusions differ in several particulars from 

 those of Nusbaum and Eeis {Bull. Acad. d. Sciences Cracovie, 1905, 

 p. 778 ; Anatomischer Anzeiger, Bd. XXXI, 1907, p. 169), I think it as 

 well to put them on record. Most of my observations were made on 

 the Pollack. If a weight be attached to this fish so as to cause the 

 gas-gland (oxygen gland) to become active and to pump oxygen into 

 the bladder, it will be found that the oval strongly contracts, so as 

 to prevent the additional gas forced into the bladder from escaping 

 into the blood. The oval, it may be mentioned, is a large oval area 

 usually situated in the dorsal posterior wall of the bladder. It differs 

 from the rest of the bladder wall in that it alone is permeable to the 

 contained gases, and, like the ductus pneumaticus in Physostomi, 

 permits their escape when the conditions require it. In the Pollack 

 the oval is normally widely open and is invisible to the naked eye, 

 but on the gas-gland being caused to become active in an unusual 

 degree, the oval becomes strongly contracted and is then a very 

 conspicuous structure inside the bladder. This contraction of the 

 oval is of course effected by muscles, and the result of it is to cause 

 the thin-walled permeable area to become more or less completely 

 shut off from the general bladder cavity, the walls of which, as just 

 mentioned, are impermeable. 



According to the observations of Nusbaum and Eeis on the ovals 

 of Perca, Lucioperca, and Ophidium, the oval has the following 

 structure : — The ordinary wall of the bladder is composed of three 



