12 



SALMONID^ OF BRITAIN, 



The head, which is comparatively large, is divided from the body by the pill- 

 openins^s, while the eye (fig. 4 e) sub-divides it into two parts, that in front of this 

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



FiR. 4. Outline of a young Lochleven trout, a. anal fin : ad. adipose dorsal fin : hi: branchios- 

 tegous rays : c. caudal tin : d. rayod dorsal tin : e. eye : i.o. iuteropercle ; l.l. lateral-line : lit. maxilla : 

 via. mandible : n. nostrils : o. opercle : j). pectoral fin : pm. premaxilla : po. preopercle : so. sub- 

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



the former is the snout wherein are placed the nostrils (n) and the mouth. The 

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

 the orbit is the infra- or sub-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 pi-eopercle (2)0). While of the two lower pieces, the posterior is 

 known as the sub-opei-cle {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 (.r) . 



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

 known as the lateral line {LI), 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 

 cither simjile and 

 undivided, or com- 

 jiosed of numerous 

 pieces articulated 



Fig. 5. Caudal ray of trout. 



to one another by 

 transverse joints, 

 and which raj's may 

 have their outer ex- 

 tremities branched. 

 A ray is essentially 



formed of two lateral halves fastened together, with the basal ends more or 

 less separated, as seen in fig. 5, which represents the caudal ray of a trout 

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

 Yin lays v.hen broken may reunite, if lost they may likewise occasionally be 

 reproduced, but often in an incomplete manner.* 



The fins are 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 (r), 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, Gobio 

 fliii-iatilis, he found tliat the fins were reproduced only when the basal portion at least is left 

 intact. Having removed tlie left ventral tin 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. (^Cumptcs liendus, March 15tb, ISO".*, t. Ixyiii, pp. CfiO-CTO.) 



