31-2 



SCANDINAVIAN FISHES. 



anal tin, and in the lai'va^ of many Teleosts its original 

 ])ositiou is so fav forward tliat in this respect it is fully 

 analogous to an anal hn. In Trarlnipferus, where no 

 distinct anal tin is developed, it has been proposed, in- 

 deed, to. regard a [lart of the caudal tin as representing 

 the remainder of the anal tin. But there is scarcely any 

 ground for this hypothesis, as in specimens 9 mm. long 

 the primary caudal tin is still perfectly normal in structure, 

 and only after this stage, though still at an early period, 



the lateral line. The latei-al line, witli its upright 

 spines — one at the middle of each plate on the hind 

 part of the tail in the older larva?" — ends at the 

 middle of the l)ase of the lower division of the caudal 

 fin, i. e. at the spa.ce between the upper of the two 

 elongated rays and the lowest of the small, middle 

 ravs of the caudal tin. 



When the larva lias attained a lenoth of 3'2 mm. 

 (fig. 80), the caudal fin has reached the highest point 



begins to undergo a considerable change, in conjunction of its development, but still retains the position common 



with a reduction of its lower (originally a.nterior) jiart. 

 When the notochord acquires its usual upwai'd curve 

 (fig. 79) — here in a double, S-shaped liend — the 

 upper (originally the posterior) [)art of the caudal fin 

 grows out in the form of a distinct division, consisting 

 of 8 rays united by the fin-membrane throughout the 

 greater portion of their length. In the lower part, on 



Fig. 80. Yours; Trai'liypterus iris, .32 lani. in length. After Emehy. 



the other hand, which consists of 6 ravs, the two rays 

 next to the lowest, which foi- the rest of the larval period 

 l)ecome considerably elongated, to about the same length 

 as that of the body, are united onh^ for a small part 

 of their length by the fin-membrane, while the other 

 rays in the lower part remain short, scarcely projecting 

 l)eyoiid the fin-membrane, the upper (innermost) three 

 resembling ordinary caudal rays, but the lowest (outer- 

 most) ray with the appearance rather of a supporting 

 I'ay. This last ray subsecjuently bends straight down- 

 wards and is changed into the doid^le spine whi<-h, in 

 the adult specimens, lies below the two last plates of 



in the Teleosts, in relation to the longitudinal axis of 

 the body. The elongated rays of the doi'sal, ventral 

 and cautlal fins are adorned with chromatophores, and 

 most of these rays are furnished Avith lobate appendages'', 

 the remnants of the fin-membrane, arranged in pairs 

 at certain intervals (fig. 81,ff). The embryonic pectoral 

 fins are still further reduced, but true rays have now 

 begun to form in the membrane. The number of rays 



Fig. 81, o. Young Tracltypterns rcpandus {iris.''), ^i^ of tin- naUiral 

 size. After CoST.\ (copied in Gunthkr). 



in the ventral fins has increased to 8, and the rays in 

 the dorsal fin extend liack almost to tlie caudal fin. 

 Along the sides of the dorsal and caudal ravs and of 

 the first ventral ray, the rudiments now begin to appear 

 of the small spines from which the geinis TraclnipterKs' 

 derives its name; but even these formations probablv 

 disappear, as a rule, at a greater age, although they 

 have also been employed as a specific character. 



The spinous plates on the lateral line and the basal 

 spines of the rays of the dorsal fin are still wanting; 

 but with these exceptions the 2'rarl/i/pferus-ty\)e is per- 

 fect, and the fish may continue to preserve this form 



" With age the plates on the anterior part of the lateral line also develop similar spines. 



* These appendages, as shown in the figure given bj- Costa and copied in GiiNruEK {Introd. SlmJ. Fish., p. 521), are prob:il)ly imtliing 

 more than remnants of the fin-membrane, torn loose as the rays detach themselves from the latter. But they are porlinps highly important 

 in a biological respect to tlie larvae. While the length of the rays assists the larva, which is without doubt a poor swimmer, to support 

 itself in the water, these processes perhaps serve to protect it by the resemblance it thus accpiires to the Stinging Medusa^ (jSip/ioiwjilionp). 

 In figure 81, b, from a somewhat older ('/., longer) specimen of Trachypterns iris from Messina, tliesu append.nges are lost, while (lie number 

 of rays is simultaneously reduced, both in the anterior dorsal and the ventral fins. 



"" "With rough fins." 



