12 



SALMONID^ OF BRITAIN, 



The head, whioh is comparatively large, is divided from the body by the gill- 

 openings, while the eye (fig. 4 e) snb-divides it into two parts, that in front of this 

 organ being termed the ante-orbital and that behind it the post-orbital region ; in 



Fig.. 4, Outline of a young Loohleven trout, a. anal fin : ad. adipose dorsal fin : hr. branoMos- 

 tegous rays : c. caudal fin : d. rayed dorsal fin : e. eye : i.o. interopercle ; 1.1. lateral-line : m. maxilla : 

 ma. mandible : n. nostrils : o. opercle : p. pectoral fin : fm. premaxilla : po. preopercle : so. Bub- 

 opercle : s.r. suborbital ring : v. ventral fin : vt. vent : x. peduncle or free portion of the tail. 



the former is the snout wherein are placed the nostrils {ri) and the mouth. The 

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

 the orbit is the infra- or snb-orbital, with its ring of bones (^s.r). The mouth is 

 somewhat oblique, and rather protractile. The gill-covers or opercles consist of 

 four pieces, the posterior and upper of which is the opercle or operculum (o) . In 

 front of it is the preopercle (^o). While of the two lower pieces, the posterior is 

 known as the sub-opercle (so), and the anterior, situated below the preopercle, as 

 the inter-opercle (i.o). 



The body or trunk, which commences just behind the head, passes by almost 

 imperceptible degrees into the caudal or tail portion, the vent (vt) separating the 

 two regions. While the finless part between the adipose or dead fin of the back 

 (ad) and the tail fin (c) is termed the peduncle or free portion of the tail (re) . 



The skin is scaled, and the row of pierced scales that passes along the side is 

 known as the lateral line (l.T), the number on which may be useful in assisting in 

 discriminating a species. 



If the composition of the fins in fishes of this family is examined, we perceive 



them to consist of 

 soft rays that are 

 either simple and 

 undivided, or com- 

 posed of numerous 

 pieces articulated 



Fig. 5. Caudal ray of trout. 



to one another by 

 transverse joints, 

 and which rays may 

 have their outer ex- 

 tremities branched. 

 A ray is essentially 

 ends more or 



formed of two lateral halves fastened together, with the 

 less separated, as seen in fig. 6, which represents the caudal ray of a trout 

 where the divided basal extremity embraces both sides of an hypural bone. 

 Fin rays when broken may reunite, if lost they may likewise occasionally be 

 reproduced, but often in an incomplete manner.* 



The fins iare divisible into such as are single and unpaired, hence termed 

 "azygous," as those along the median line of the back or dorsal (fig. 4, d and ad). 

 The tail or caudal fin (c), placed vertically at the posterior extremity of a fish 

 where it may be seen of two very distinct types ; in the generality of the finny 

 tribes when the two lobes are equal it is termed " homocercal," as in the perch or 

 carp ; whereas in the sharks and some other allied classes the vertebral column is 



* According to M. M. Philipeau's investigations and experiments made on gudgeons, GoUo 

 fluviatilis, he found that the fins were reproduced only when the basal portion at least is left 

 intact. Having removed the left ventral fin on a level with the abdominal surface, it became 

 completely restored in eight months. In another example he repeated the operation, also taking 

 away the small bones which supported it, but at the end of eight months there was no trace of the 

 fin's regeneration. (C<mvptes Bendus, March 16th, 1869, t. Ixviii, pp. 669-670.) 



