UOIJUE-MACKERELS. 



85 



out transverse branches on each side, lint in the la- 

 teral line proper the traiisxerse branches of the. lii'st 

 magnitude (;i pair I'oi- e;ich plate, one upwards nnd 

 one downwards) send out only b;iek\\;irds a number 

 (from 5 to 7) of secondary branches", which open into 

 the posterior nuirgin of the plate. In older specimens 

 (jf this species, in which these canals often become in- 

 distinct, we m;iy still hud numerous, scattered pores, 

 especially on the back and head, which are the orifices 

 of the system of the laternl line. Of the other species 

 of the genus, Caraiix torrus lias the dorsal canal simi- 

 larh- contimied almost to the end of the second dorsal 

 hn, l)ut in ('(ir. armatus ^ve liave not been able to trace 

 it farther than the beginning of the first dorsal, and 

 in some specimens of Car. lepturus it may be traced 

 to the end of the second dorsal, in others only to the 

 beginning of the first, as is also the case in the genus 

 Selene. The variations in this respect indicate the 

 transition to Temnodon and the true Mackerels, some 

 of which liave onlv a rudimentary dorsal canal, while 

 others are \\'ithout it altogether. 



In Caranx, as in Selene, Trachynotus and Licliia 

 (I am luiable to speak -with certainty of the other Ca- 

 rangidm), the first three interneural bones are separate 

 from each other, while the others are more or less clo- 

 sely united by the osseous ridges on their back and front. 

 The coalescence is most complete between the 4th and 

 5th. Fi'om the 4th grows out in a forward direction 

 the spine mentioned in the diagnosis, and the first spi- 

 nous ray in tlie anterior dorsal fin is supported by the 

 same Ijone. On each of the first three interneural bones 

 there is a process in a forward direction Avhich corre- 

 sponds to the spine just mentioned. These processes 

 do not, however, project above the skin, though the 

 tops of these three bones are sometimes visible as oblong 

 knobs in the edge of the back in front of the first 

 dorsal fin. 



The genus Caranx has gained some further interest 

 in an anatomical respect from an observation the 

 universal signification of which within the genus is, 

 however, still unknown. The chief function of the air- 

 bladder is, as is well known, to enable the body of the 

 fish to maintain or attain the same specific gravity as 



that of the surrounding water. Thus, it is more com- 

 yn'essed (the size of tlie fish smaller), but contains more 

 gas (of greater density) in dec]) water than near the 

 sui'f'ace. When the fish gi'adiiajjy rises or sinks in the 

 water or only slightly changes its position, the quantity 

 of gas in the air-bladdtn- is altercid in proportion. When 

 the fish is rising and it is necessary to diminish the 

 quantity of gas, the Physostomi, whose air-l)Iadder is 

 furnished with a pneumatic duct, thus enjo}' a great 

 advantage, for those fishes which have no pneumatic 

 duct, can only gradually diminish the (piantity of gas 

 by absorption. Hence, too, the great ditt'ei'ence in the 

 effect upon different fishes of a, change in the pi-essurc 

 upon the water, e. g., when the pressure is diminished. 

 Those which have no air-ljladder, seem but very slightly 

 affected by the change; those ^vhich have an air-bladder 

 with pneumatic duct, emit gaseous bubbles through the 

 mouth and gill-openings, but those which have an air- 

 bladder without pneumatic duct, are drawn upwards in 

 spite of their struggles to sink deeper and deeper, until, 

 if they find it impossible to descend, they are quickly 

 borne to the surface, where they lie swollen up and 

 unable to move. In his endeavours fully to investigate 

 this point Arm and Moreau'' found that Caranx trachu- 

 rus, far from being borne to the surface like the Basse 

 and the other Phj'soclysts experimented on, when the 

 air above the vessel in which it Avas placed, was rarefied, 

 seemed only slightly uneasy and emitted only small 

 gaseous bubbles. (Jn closer examination he found in 

 the upper wall of the air-bladder, in a line with the 

 seventh rib, an opening surrounded b}' a raised, val- 

 vular margin and leading to a tube which follows the 

 right side of the aorta in a for^vard direction and after 

 a geniculate curve opens into the right gill-cavity. 



Another peculiarity, the universal signification of 

 which is still to be decided, is the fact that the fiy of 

 Caranx trachuriis is a parasite of the Medusa^ a cir- 

 cumstance which is one of the many discoveries made 

 by the late Professor A. W. Malm (see below). 



The numerous species of this genus are excellent 

 swimmers and thus, for the most part, have a wide 

 range. They are therefore subject, as is usual under 

 such circumstances, to fairly many variations of form. 



" They may be seen in McCoy's figure (Prod. Zool. Vict., pi. XVIII), us it is reproduced in Fish. a. Fisher. X. 6'. \V<iles by 

 Tenison-Woods (Sydney 1882) pi. XXII, fig. 1 a, showing the inside of a plate from the Australian Car. trachurus, . nUch figure may be 

 compared with our figure of the high scales in Pterycninbus brama. 



* Ann. Sc. Nat., ser. VI, Zool., Tome IV (1876), Art. No. 8. 



