HEAD, MOUTH AND GILL-COVERS. 



vu 



into a weapon of offence as in the sword-fish (plate xlix), or the gar-fish 

 (plate cxxvi), while the tail-fin may be elongated for a similar purpose as 

 seen in the fox-shark (plate clvii), or the tail may be capable of prehension 

 as in horse-fishes (plate cxliv, fig. 7) . Some are possessed of organs for 

 attachment as seen on the top of the head in the sucking-fish (plate xxxix), 

 on the chest as in the lump-sucker (plate lv) and Liparis (plate lvi), or the 

 mouth may be formed for. this purpose as in the lampreys (plate clxxviii). 

 In Asia we see a few carps have an adhesive organ behind the lower jaw, as 

 in Discognathus, while some sheat-fishes as Pseudecheneis have them on the 

 chest, thus enabling them to reside in mountain torrents or rapid streams. 



Fig. 1. Red Mullet. 

 h, maxilla ; c, mandible ; o, barbel ; h, pectoral fin ; i, ventral fin ; It, first dorsal fin ; 

 Z, second dorsal fin ; m, anal fin ; n, caudal fin ; q, lateral-line ; p, lateral-traneverse line. 



The head is as a rule divided from the body by the gill-openings, while the 

 eye sub-divides it into two parts, that in front of this organ being known as 

 the ante-orbital and that behind it as the post-orbital region : in the former 

 is the snout wherein are placed the nostrils and generally the mouth. The 

 space between the orbits is known as the inter-orbital region, while that 

 below the orbit is the infra- or sub-orbital. 



The mouth is subject to great diversity in its external direction, being 

 either horizontal, oblique, or almost vertical. In some it is' semi-circular or 

 even circular in the lampreys, and inferior in most of the sharks. It may 

 be very or only slightly protractile, and possess or be deficient in lips : and 

 sometimes provided with accessory tactile organs in the form of barbels 

 (fig. l,o). 



The gill-covers or opercles in most bony fishes, consist of four pieces, the 

 posterior and upper of which is theopercle or operculum (page xv, fig. 3, 0), 

 in front of it the preopercle (fig. 3, p. 0), while of the two lower pieces 

 the posterior is known as the sub-opercle (fig. 3, s. 0), and the anterior, 

 situated below the preopercle (and sometimes rudimentary or absent), as the 

 inter-opercle (fig. 3, i. 0). Although in telcosteans, ganoids and holocephala 



