446 C. JUDSON HERRICK 



air breathing is not so evident. Nevertheless, this collocation 

 seems to have been actually realized. I am not prepared to 

 offer a satisfactory solution of this problem, though the following 

 suggestions may be presented for further consideration and 

 criticism. 



For fishes living in imperfectly aerated water the oxygen 

 supply of the brain is a vital matter. The arteries of the brains 

 of fishes are derived from the internal carotid, which brings 

 freshly aerated blood directly from the gills (see the description 

 of the cerebral arteries of Ceratodus by Bing and Burckhardt, 

 '05) . This vascular supply to the brains of lungfishes is very rich, 

 as indicated by the following comment by Graham Kerr ('02, 

 p. 428) : 



In dissecting the brain of Lepidosiren one is struck by the ex- 

 traordinary development of richly ramifying blood-vessels within the 

 cranial cavity, forming a packing all round the brain. This may pos- 

 sibly be an adaptation to the times at which it is impossible to make 

 the blood rich in oxygen, during the final stages in drying up of the 

 swamps, or during casual rainfalls in the dry season. 



The brains of lungfishes and all of the more generalized ganoids 

 are, moreover, provided with extraordinarily enlarged choroid 

 plexuses which probably ensure the highest possible oxygen 

 supply to the cerebrospinal fluid within the brain ventricles 

 and in the surrounding endocranial spaces. Obviously, this 

 arrangement is well adapted to make the largest possible use of 

 a deficient supply of oxygen, for the nervous tissues can take up 

 oxygen directly from the blood-vessels which envelop and pene- 

 trate their mass and also from the cerebrospinal fluid by which 

 they are bathed. But the latter source of oxygen is available 

 only on the ventricular and external surfaces, and the interior 

 of any considerable thickenings such as characterize the fore- 

 brains of teleosts would be dependent upon a single source of 

 oxygen only, viz., the penetrating blood-vessels. This latter 

 source is obviously adequate for species hving in well aerated 

 water, but for mudfishes, and especially those subjected to periods 

 of drought, the interior of such thick masses might suffer asphyxi- 



