39Q FISHES 



bladder, and receive their blood supply from a branch of the 

 coeliac artery, the corresponding vein joining the portal veins 

 so that the returning blood passes with the blood from the 

 digestive organs to the liver. In fishes with closed bladders, 

 for example the maigre, Scicena aquila, the red body consists of 

 a dense collection of capillaries, above which is a thick mass of 

 epithelial glandular cells : the vascular layer belongs to the 

 inner of the three layers which form the wall of the bladder, 

 the origin of the glandular mass is not known. Above the 

 organ extends the layer of flat cells which everywhere lines the 

 bladder. In Scicena the glandular mass contains rounded 

 cavities communicating with the surface by narrow channels : 

 in other cases the glandular mass is thinner, and contains no 

 cavities. According to Jaeger and Dr. Woodland to whom we 

 owe the most important of recent investigations of the subject, 

 the gland obtains oxygen from red blood corpuscles brought 

 by the capillaries, and excretes it with some force into the 

 bladder. This is the explanation of the fact that the pressure 

 of the oxygen in the bladder is so much greater, especially in 

 fishes which live at considerable depths, than what is called 

 the tension of the gas in the blood. 



The sense-organs are necessarily adapted to their special 

 functions, and in fishes they present some very curious modi- 

 fications. 



According to the American zoologist G. H. Parker, there 

 is no conclusive evidence that the sense of hearing exists at all 

 in the majority of invertebrate animals, such as coral polyps, 

 jelly-fishes, worms, star-fishes, crabs, oysters, snails, etc. These 

 animals possess organs which resemble auditory organs, and 

 were formerly so described, but recent experimental researches 

 have shown that their function is the sense of equilibrium, a 

 sense which is associated with the auditory organ in the verte- 

 brates also. The only animals in which the sense of hearing 

 is known with certainty to be present are the higher arthropods, 

 especially the insects, and the vertebrates. The fact that the 

 sense is certainly most developed in the animals which live in 

 air suggests the question whether the sense really exists in 

 aquatic animals, and in the case of vertebrates this question 

 must be tested by the investigation of the sense in fishes. 

 Bateson, from his observations at Plymouth, found that the re- 



