312 
SCANDINAVIAN FISHES. 
anal fin, and in the larva? of many Teleosts its original 
position is so far forward that in this respect it is fully 
analogous to an anal fin. In Trachypterus, where no 
distinct anal tin is developed, it has been proposed, in- 
deed, to regard a part 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 fin is still perfectly normal in structure, 
and only after this stage, though still at an early period, 
begins to undergo a considerable change, in conjunction 
with a reduction of its lower (originally anterior) part. 
When the notochord acquires its usual upward curve 
(fig. 79) — here in a double, S-shaped bend — the 
upper (originally the posterior) part 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 
the other hand, which consists of 6 rays, the two rays 
next to the lowest, which for the rest of the larval period 
become considerably elongated, to about the same length 
as that of the body, are united only for a small part 
of their length by the fin-membrane, while the other 
rays in the lower part remain short, scarcely projecting 
beyond the fin-membrane, the upper (innermost) three 
resembling ordinary caudal rays, but the lowest (outer- 
most) ray with the appearance rather of a supporting 
ray. This last ray subsequently bends straight down- 
wards and is changed into the double spine which, in 
the adult specimens, lies below the two last plates of 
the lateral line. The lateral line, with its upright 
spines — one at the middle of each plate on the hind 
part of the tail in the older larvae" — ends at the 
middle of the base of the lower division of the caudal 
fin, i. e. at the space between the upper of the two 
elongated rays and the lowest of the small, middle 
rays of the caudal fin. 
When the larva has attained a length of 32 mm. 
(fig. 80), the caudal fin has reached the highest point 
of its development, but still retains the position common 
in the Teleosts, in relation to the longitudinal axis of 
the body. The elongated rays of the dorsal, ventral 
and caudal tins are adorned with chromatophores, and 
most of these rays are furnished with lobate appendages * 6 , 
the remnants of the fin-membrane, arranged in pairs 
at certain intervals (tig. 81, a). 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, a. Young Trachypterus repandus (iris?), '/ 3 of the natural 
size. After Costa (copied in Gunther). 
in the ventral tins has increased to 8, and the rays in 
the dorsal fin extend back almost to the caudal fin. 
Along the sides of the dorsal and caudal rays and of 
the first ventral ray, the rudiments now begin to appear 
of the small spines from which the genus Trachypterus c 
derives its name; but even these formations probably 
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 Trachypterus - type is per- 
fect, and the fish may continue to preserve this form 
u With age the plates on the anterior part of the lateral line also develop similar spines. 
6 These appendages, as shown in the figure given by Costa and copied in Gunther (Introd. Stud. Fish., p. 521), are probably nothing 
more than remnants of the fin-membrane, torn loose as the rays detach themselves from the latter. But they are perhaps highly important 
in a biological respect to the 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 acquires to the Stinging Medusae ( Siphonophorce ). 
In figure 81, b, from a somewhat older ( l / 3 longer) specimen of Trachypterus iris from Messina, these appendages are lost, while the number 
of rays is simultaneously reduced, both in the anterior dorsal and the ventral fins. 
c “With rough fins.” 
