FISHES. 33 



many fishes ; the red, apparently glandular body, that in most bony 

 fishes surrounds the entrance of the optic nerve into the ball of 

 the eye at the choroid coat [glandula choroidalis) , is merely such 

 a vascular network. 



The respiratory organs of fishes, the gills, consist in most of 

 small leaflets usually triangular and of equal breadth, attached by 

 their bases to the branchial arches on which the branchial arteries 

 ramify. These leaflets mostly form two rows on each branchial 

 arch ; if one such row alone be present, then the name of half-gill 

 is given to such an arch. In the bony fishes and the sturgeons the 

 branchial arches are on the outside free; the water is taken in by 

 the mouth and afterwards expelled by two gill-apertures behind 

 the gill-covers (one on each side). In the sharks and rays, on the 

 contrary, there proceeds from each branchial arch, between the 

 branchial leaflets of the anterior and posterior row-, a membranous 

 production as far as the skin, entirely covering the gills in these 

 fishes, excepting five apertures for the expulsion of the water; 

 hence arise complete partitions between the pharynx and the skin, 

 in which the branchial arches are situated. The posterior wall of 

 the fifth gill-cavity is here without branchial leaflets. The leaflets 

 are cartilaginous internally, or rarely bony, and are covered with 

 a rich vascular net, so as to have a bright red colour'. The entire 

 apparatus is covered by a continuation of the mucous membrane of 

 the mouth. In the respiration of fishes the air contained in the 

 water is alone effectual, and by no means the oxygen of the water, 

 which is not decomposed by this function. Some fishes mount to 

 the surface of the water to breathe atmospheric air itself, and they 

 die when this is rendered impossible by gauze interposed. But 

 fishes cannot live long out of water ; some die even very rapidly, 

 because the leaflets fall together and cohere, so that the circulation 

 of blood in these organs is interrupted, and the oxygen of the air 

 cannot act upon the parts that thus cover each other ^. 



With the half-gills or the accessory gills, which in the stur- 

 geons occur on the gill-cover, those false gills [fseudo-brancMcR) 



1 DOELLINGER gave a very beautiful figure of thia vascular net in Ahhandl. der 

 mathem. physik. Klasse der Ahad. zu Miinchen, ii. 1837. Ueher die Vertheilung des 

 Mutes in den Kiemen der Fische, s. 785 — 794, Tab. i. fig. 3. 



^ See Flourbns Exj)eriences sar h mecanisme de la respiration des Poi.<;sons, A nn. 

 des )Sc. nattir. xx. 1830, pp. 5 — 25. 



VOL. II. 3 



