190 LECTURE VIII. 



The Loplnus lurks at the bottom, hidden in the sand, waiting, like 

 the Skate, for its prey to come within the reach of its jaws : the 

 difference in the shape, size, and structure of their prosencephala is 

 hardly less than that between the Shark and Pike. The combative 

 Stickleback has longer and narrower prosencephala than the cowardly 

 Gudgeon. The nidificative and philo-progenitive Callichthys * has 

 neither the antero- lateral nor the posterior regions of the cerebrum 

 more developed than in bony fishes generally. 



MEMBRANES OF THE NEURAL AXIS. 



Both brain and myelon are immediately invested by a thin but 

 firm fibro-cellular and highly vascular membrane, the outer surface 

 of which is usually covered by a stratum of pigment-cells, belonging 

 properly to the central layer of the arachnoid, which has here co- 

 alesced with the proper vascular pia mater. This vascular membrane 

 seems, therefore, to be coloured with dark points, and sometimes to 

 be minutely speckled upon a silvery ground ; and the pigmental 

 stratum often accompanies the processes of the pia mater in the ven- 

 tricles of the brain. There is commonly a remarkable development 

 of the vascular and pigmental membrane over the fourth, or epen- 

 cephalic ventricle. I found such a mass concealing the rudimental 

 cerebellum in the Lepidosiren ; it is largely developed in the Sturgeon 

 and Paddle-fish, where it is posterior to the cerebellum. The 

 commonly considerable space between the brain and cranial walls 

 is occupied by a peculiar loose cellular structure, filled by gelatinous 

 or albuminous fluid, and by oily matter : in the Perch and Bream it 

 seems to consist of an aggregate of minute spherical cells filled with 

 fine colourless oil, the mass being traversed by blood-vessels. Cuvier f 

 found the cells, which he compares to a kind of arachnoid, filled by a 

 pretty compact adipose matter in the Tunny and Sturgeon. This 

 modified arachnoid exists, but in less quantity, in the spinal canal, 

 and even accompanies the cerebral nerves in their exit from the skull 

 in some fishes with large nerve-foramina. The quantity of the 

 cellular arachnoid above the cerebral lobes of Lepidosiren is a striking 

 example of the piscine nature of that genus. 



The primitive fibrous capsule of the neural axis, the unossified or 

 unchondrified remains of which, or of its inner layei', form the so- 

 called ' dura mater,' is most distinct in the low-organised Dermopteri; 



* Callichthys litloralis, or Hassar-fish of Demerara, See the specimen of its 

 brain, No, 1309, h, and that of its nest and eggs, No. 3787, h. Phys. Series, and 

 an account of its habits in the Zoological Journal, vol. iv. p. 244. 



f XXIII. i. p. 309. 



