222 DR. W. N. F. WOODLAND ON THE 



part of tlieir blood-supply was cut off. It is also certain that 

 most physostomous fishes do not obtain their gases from the 

 atmosphere vid the pneumatic duct, since the pneumatic duct is 

 iiow known to be merely one of the two principal mechanisms 

 (the other being the " oval ") employed by fish for the elimination 

 and not the obtainment of gas. Finally, Thilo's theory fails to 

 account both for the degeneration of the ductus pneumaticus in 

 Physoclisti (since, according to Thilo, it is still functional) and 

 for the presence of the complex "red bodies" which are the very 

 structures needed by the fish on account of the feeble blood- 

 circulation referred to by Thilo. 

 .^ Theories which derive the bladder gases from the blood stream 

 can again be grouped into two classes, viz. those which suppose 

 that the blood gives up its gaseous constituents more or less 

 directly to the bladder, the gas passing straight from the 

 capillaries of the rete miiubile, or wall of the bladder, into the 

 bladder lumen, and those which regard the gas gland as the 

 special organ which extracts the gas from the blood. The first 

 class of opinions, to some extent associated with the name of 

 Moreau, is now quite out of date, though still to be found stated 

 in some recent text-books. The second class of opinions comprises 

 two quite distinct views as to the exact function of the gas gland 

 - — two views which at the present time are held with equal tenacity 

 by the schools represented by Jaeger and Nusbaum & Reis 

 respectively. I shall first state briefly the view of Jaeger (44-48). 

 Jaeger, following Hiifner (38), holds that the gas gland is 

 pi-imarily a pumping apparatus, that is to say, an appai'atus for 

 pumping the gases contained in the blood into the bladder cavity. 

 The pressure exerted by the gases in the blood is, of course, 

 considerably less than that exerted by the gases in the bladder *, 

 and it is the function of the gas gland to force the gas from the 

 blood into the bladder lumen against this superior pressure. 

 Jaeger further supposes that the disintegration of a certain 

 percentage of red blood corpuscles is effected by the secretion on 

 the part of the gas gland cells of a toxin which is poured into the 

 blood for this purpose. The object of so breaking up the ery- 

 throcytes is to enable the gas gland cells, in some way not 

 described, to lay hold of, with greater facility, the oxygen thus 

 scattered in the corpuscle fragments. Indeed, Jaeger, following 

 "Moreau, regards the gas gland as a mechanism essentially con- 

 cerned with the pumping of oxygen — as an oxygen gland, in 

 short, — a view confirmed both by the great development of this 

 gland/ and by the disintegration of the er-ythrocytes in connection 

 with bladders containing a large percentage of oxygen, and aJso 

 by the contrary fact that in the bladders of Cyprinoids and many 

 other freshwater fish which mostly contain nitrogen, the gas 

 glands are absent — the ordinary squamous epithelial lining here 

 being capable, without undergoing any special modification into a 



* JE. g. Jaeger (46) states that in deep-sea fish the partial pressure of the ox3-geii 

 in the blood only amounts to about one-iifth of an atmosphere, whereas the oxygen 

 in the bladder may possess a partial pressure of over fort}' atmospheres. 



