GAS GLANDS OF SOMli XiiLEOSTEAN FISHES. 221 



directly fi'om the blood stre.im. The first view, abandoned by 

 neai-ly all modern authors, is, however, persistently maintained 

 by Tlulo(70, 71, 72). Thilo's principal contention, stated briefly, 

 is that the blood of a fish is not suflicient in quantity to contain 

 the amount of gas found in the bladder, and that even if this 

 were the case the circulation of the fish is so feeble that the blood 

 could not replenish an emptied bladder in the time experiment 

 proves that it can be replenished. He therefore asserts that in 

 all cases the bladder-gas must have been procured by the fish 

 directly from the atmosphere : Physostomi can always renew their 

 bladder -gas by rising to the surface of the water and passing air 

 along the ductus pneumaticus, whilst Physoclisti are apparently 

 under the hard necessity of absorbing a suificient supply when 

 young, and therefore before the duct has degenerated, to last 

 them throughout life, though Thilo further maintains that in 

 many cases even adult Physoclisti are still able to procure fresh 

 supplies b}^ passing air, either swallowed from the atmosphere or 

 extracted in some inexplicable manner from the water, along the 

 strand of tissue representing the vestigial duct. Thilo performed 

 experiments and, according to his statements, obtained results 

 which strongly support his view. He asserts, e, g. that he cut off 

 the vascular supply of the previously-emptied bladder in Tinea (a 

 physostome) and that in thirty hours it became refilled with gas. 

 Seeing that the blood-supply was absent, he argixes that this fresh 

 supply of gas must have been obtained from the atmosphere by 

 passage thi-ough the pneumatic duct. I, however, cannot find 

 that he analysed the gas produced under these conditions, and, 

 since atmospheric air and secreted gas nearly always consist of 

 oxygen, nitrogen and carbon dioxide associated in very different 

 proportions in the two cases, his contention as to the soiirce of the 

 gas receives very little real support from his expei-iments. A few 

 of the many obvious objections to Thilo's hj^pothesis may be 

 stated. In the first place, the percentage composition of the 

 three gases contained in the bladder is, as just mentioned, quite 

 diflerent from that present in the atmosphere, and in the cases 

 of oxygen and nitrogen these gases are often present in such 

 quantities as to exert a pressure many times greater than the pres- 

 sures they exert in air — both of which elementary facts are fatal 

 to Thilo's view. Thilo's contention that the blood of a fish with 

 its feeble circulation is incapable of providing the quantity of gas 

 required by the bladder of course begs the question at issue, and 

 he might with equal force contend that the tissues of a siliceous 

 sponge, for example, must be incompetent to deposit a massive 

 siliceous skeleton, seeing that 100,000 parts of sea -water contain 

 little more than one part of silica in solution and that diatoms 

 a,nd other minute organisms are serious comj)etitors even for this 

 small quantity (Sollas). Fiu^ther, Thilo's experimental results are 

 not in accordance with those obtained by Hufner(38), Jaeger (45), 

 and others, and, indeed, Thilo's own experiments failed to render 

 his contention even pi'obable since the gland cells of the bladder 

 would not necessarily cease their activity immediately a large 



