622 



PHYLUM CHORDATA : CLASS JPISCES — FISHES 



Nervous system. — The relatively undifferentiated fore- 

 brain with defective cortical region, the thalamencephalon 

 with its inferior lobes and infundibulum, the large optic 

 lobes, the tongue-shaped cerebellum which conceals most 

 of the medulla oblongata, have their usual general relations. 

 Each of the olfactory nerves is at first double ; their bulb- 

 like terminations lie far from the 

 brain behind the nasal sacs. The 

 large optic nerves cross one another 

 without fusion at a slight distance 

 from their origin ; otherwise the 

 nerves generally resemble those of 

 the skate. 



The eyes are large but lidless ; 

 the small nasal sacs with plaited 

 walls have double anterior apertures; 

 the vestibule of the ear contains a 

 large solid otolith, and another very 

 small one in a posterior chamber. 

 The dark lateral line, covered over 

 by modified scales, lodges sensory 

 cells, and is innervated by a branch 

 of the vagus. 



Alimentary system. — Teeth are 

 borne by the premaxillae, the vomer, 

 and the superior pharyngeal bones 

 above, by the dentaries and the 

 inferior pharyngeal bones beneath. 

 There are no salivary glands, no 

 spiracles, nor posterior nares. A 

 small non-muscular tongue is sup- 

 ported by a ventral part of the hyoid 

 arch. Five gill-clefts open from 

 the pharynx ; their inner margins are 

 fringed by horny gill-rakers attached to the branchial arches 

 and serving as strainers ; they prevent the food from being 

 swept out with the respiratory current. The gullet leads 

 into a curved stomach ; at the junction of stomach and 

 duodenum numerous tubular pyloric caeca are given oft" ; 

 into the duodenum opens the bile-duct from the gall- 

 bladder and liver ; the coiled intestine passes gradually 



Fig. 357. — Section of a 

 Teleostean gill. 



G.F., Gill-filament; A., artery 

 (venous blood) ; V., vein (pure 

 blood) ; G.A., gill-arch. 



