38 FISHES, 



of each side being more or less close together. They lead into 

 a shallow groove; and only in one family (the Myxinoids) 

 perforate the palate. In -this family, as well as in the Lam- 

 preys, the nasal aperture is single. In many Eels the open- 

 ings are lateral, the lower perforating the upper lip. In the 

 Sharks and Pays (Fig. 1, p. 34) they are at the lower surface 

 of the snout, and more or less confluent ; and, finally, in the 

 Dipnoi and other Ganoids, one at least is within the labial 

 boundary of the mouth. 



The space across the forehead, between the orbits, is called 

 the inter-orhital space ; that below the orbit, the infra-orbital 

 or sub-orhital region. 

 Gill-cover. In the post-orhital part of the head there are distinguished, 

 at least in most Teleosteous Fishes and many Ganoids, (Fig. 

 24) the prceoperculum, a sub-semicircular bone, generally with 

 a free and often serrated or variously-armed margin ; the 

 operculum, forming the posterior margin of the gill-opening, and 

 the suho2Jercidum and inter-op)erculum along its inferior margin. 

 All these bones, collectively called opcrdcs, form the gill-cover, 

 a thin bony lamella covering the cavity containing the gills. 

 Sometimes they are covered with so thin a membrane that 

 the single bones may be readily distinguished; sometimes 

 they are hidden under a thick integument. In some cases the 

 inter-operculum is rudimentary or entirely absent (Siluroids). 

 Gill- The Gill-opening is a foramen, or a slit behind or below the 



opening. ]^ga(j^ |3y -vvliich the water which has been taken up through 

 the mouth for the purpose of breathing is again expelled. 

 This slit may extend from the upper end of the operculum all 

 round the side of the head to the symphysis of the lower jaw; 

 or it may be shortened and finally reduced to a small opening 

 on any part of the margin of the gill-cover. Sometimes 

 {Symhranchus) the two openings, thus reduced, coalesce, and 

 form what externally appears as a single opening only. The 

 margin of the giU-cover is provided with a cutaneous fringe, 



