90 



SCANDINAVIAN FISHES. 



orbital bone. Both grooves serve to receive the upper 

 jaw when the mouth is closed. For this purpose in 

 the Tunnies there is an expansion of the furro-w in the 

 lower ja^v at the corner of the mouth, Avhere the broad 

 posterior part of the upper jaw may rest. This ex- 

 pansion is fairly great even in Anxis. In the true 

 Mackerels the upper jaw-bone slides completely under 

 the preorbital bone, when the mouth is closed, and 

 thus the expansion of the groove in the lower jaw is 

 inconsiderable. 



Of the air-bladder there is nothing to remark, 

 save that it is sometimes wanting in species very closely 

 related to others where it is present. This family, 

 however, like the following, is remarkable for the power 

 which some of its larger members possess, of raising 

 their own temperature considerably above that of the 

 surrounding ^vater. Davy states" that on his voyage 

 to Ceylon he fdund the temperature of l)ody in a Bonito 



— "in the deep-seated muscles in the thickest part of 

 the hsh, a little below the gills" — to be 99° Fabr., 

 while the temperature of the water at the surface 

 was only 80.5°. He was also informed by fishermen 

 that they had observed the same circumstance in the 

 Tunny and in other large species of this family*. This 

 increase of temperature is connected with the great 

 muscular activity enjoyed b}' the Scombroids. This mus- 

 cular activity has its physiological conditions in an 

 abundance of richly corpuscular blood, a powerful'' 

 respiration and circulation, a thick, muscular heart and 

 a strong innervation not only of the muscles of the 

 body, but also of tlie organs of respiration. Thus Davy 

 found the basal ganglia of the gills {ganglion hmnchiale, 

 belonging to nerrus ragus) to be larger in the Scom- 

 broids than in other fishes and almost as large as the 

 "electric ganglia" in the Toi'[)edo. The quick and ready 

 movements of these fishes are also aided by the reduc- 

 tion of the scales of tlie body which generally occurs 

 in this family. Tlie scales are sometimes fairly large, 

 as in the region of the pectoral fins, and cycloid or 



— as in Scoml/er kunaguyta from the Red Sea and the 

 Indian Ocean — with wavy striations and crenulated 

 at the margin. Often, hoAvever, they are wanting, at 

 least externally, either o^■er the -whole body or over the 



greater part of its posterior portion, or else they are 

 imbedded in the skin, as on the cheeks, where, as well 

 as in the corslet, they are generally larger than on the 

 rest of the body. All the Scombroids belong to the 

 most active of salt-Avater fishes, and their power of 

 wandering so far is closely connected with their wide 

 geographical range. 



About 70 species, all belonging to the seas of the 

 Tropic and Temperate Zones, are recognised and de- 

 scribed in Gunther's Catalogue; but several of these 

 species are only imperfectly known, and on closer ob- 

 servation, as LuTKEN has shown with regard to the 

 Tunnies, will probably be explained as alterations due 

 to age or varieties of other species. The number of 

 forms within the family is large enough, however, to 

 admit of sharp disputes not only in the distinction of 

 the species, but also in their arrangement in genera. 

 Though tliis may depend to a great extent on our 

 ignorance of changes due to age in the dift'erent spe- 

 cies, an additional reason is the considerable size of 

 some of the species, which renders it difficult clearly 

 to understand the relations of form. All that we know 

 of the changes due to age, is summed up by Lutken'' 

 in some observations on the spines of the preoperculum, 

 the larger jaw-teeth, and the shorter maxillaries of the 

 fry (in Scomher colias). He also describes some small 

 fishes from 8 to 17 mm. in length, Avhich he refers 

 with good reason to some species of Tunny, and assu- 

 mes this to be Orcynus germo. These small fishes 

 (fig. 25) are short and compressed in form, with sharply 



Fig. 25. Young Tunny, probably Orcynus germo. 

 3 times the natural size. After Lutken. 



dentated preoperculum and with the second dorsal 

 and the anal fins continuous, but with signs of the 

 breaking-up of the posterior part into finlets. The 

 mouth is large, its aperture, when shut, rising at an 

 angle of 45°. The pectoral fins are broad and short, 

 and the ventral short and rounded. With regard to 

 the continuation of the changes of form during growth, 



" K.liiil., N. Phil. Jc.nrn., vol. XIX (1835), p. 325. 



* Cf. also Caupenteu: Man. FJii/siol., p. 481; Couch: Fish. Brit. lal.. vol. II, p. 95; McCoy: Prodr. Zool. Vict., Dec. V, p. 24. 

 ' The more complicatotl structure of llio gills in the Swordfislics and in one genus of the Mackerel family {Acanthoctjbium), where the 

 sevcriil branchial laminaa coalesce into one lamina, meshed like a net, is also probably connected with this circumstance. 

 '' ^polia Atlantica, 1. c, p. 482. 



