SUNFISHE.S. 



628 



occasions, and tlie late developniunt of the caudal fin 

 has been fully demonstrated (fig. 154). According to 

 Lltkkn ", however, we have a still earlier developmental 



Fig. 154. 



Liirv* of Orthaijori/tcns iiiola; A. .it a length of 18 mm. 

 B. at a lengtli of 'A2 mm. After GCnther. 



stage of Orthagoriscus in the form n.amed bj- Richard- 

 sox Ostraclon hoops (fig. lo.'i), after the larva] speci- 

 mcn.s taken at the surface of the South Atlantic and 



Fig. 155. 



/jOOpS, RlCHARPSOX. 

 After Gf.NTHER. 



Ponxrfully iiuignified. 



figured l)y Hookeh. These larvie are of a still more 

 curious form, with their large eves and their equipment 

 of large spines, such as those we have seen above on the 

 heads of several Acanthopteiygian larvas, notably within 

 the great series of the Scombromorphs, and with small 

 spines besides on the dorsal side. The diffei'ence i)e- 

 tween them and Kolreuter's larva, however, is not so 

 great as to prevent us from easily recognising the cor- 

 ivsponding position of the large spines. As in the 

 younger of Kolredtek's larviB we here find one large 

 spine at the middle of the forehead between the eyes; 

 three at the sharp dorsal margin, the hindmost of which, 

 just in fi'ont of the dor.sal fin, is the largest; one' on 

 each side above the eyes, as well as on each side of 

 the hind part of the body between the dorsal and anal 

 fins. \\'hether the other spines correspond exactly in 

 both larva}, is a more difficult point to decide. The 



caudal tin is still less developed than in the younger 

 of Kolreuter's lai'vu'. .\s tiie develo])ment advances, 

 the large spines are said to bet-ome <om|iaratively 

 smaller, but the small spines, tiie papilke with radiat- 

 ing striie, to extend densely and evenly over the whole 

 body. The transition to the Orfhaf/orisciis form is thus 

 not far distanf. 



If we compare these youngest forms of Orthaqo- 

 riscKS with the normal forms of piscine larvie, the 

 greatest difference lies in the fact that here the caudal 

 fin, which otherwise is the first fin to appear — as it 

 also is the original organ of motion in all vertebrates — , 

 is developed last of all the fins. This difference has 

 left its mark not only on the structure of the skeleton, 

 but also on the development of the muscles. Here the 

 original and, in later life, the most powerful organs of 

 motion are the dorsal and anal fins. Their supporting 

 bones (the interneural and interhEemal bones) and their 

 motory mu.scles exercise a determinant influence on 

 the development of the whole trunk- 



In dissecting an Orthnfforisciis'' we have first to cut 

 through the enormoush" thick skin, which is lined with 

 a strong tendinous membrane {apotieuros). Within the 

 latter, on the sides of the body, we make the i-emark- 

 able observation (fig. 15(5) that the muscles most highly 

 developed in other fishes, the large lateral muscles of 

 the l)odv, are here apparently wanting. Between the 

 spinal column and the skin runs, as usual, in a hori- 

 zontal dii'ection, a fibrous membrane (sf), which here 

 is especially strong, and divides the visible muscular 

 mass, just as in other cases it divides the large lateral 

 muscles of the body, into two halves, an upper and a 

 lower. P>ut b\- far the greater part of this muscular 

 mass is here composed of the flexors of the dorsal (mpd) 

 and anal (nqta) fins. The flexors of the dorsal fin, 

 divided into two layers of different colour and firmness 

 — a surface-layer and an nnder-layer — run from the 

 occiput, from the spinal column with its superior pro- 

 cesses and interneural bones, and from the upper sur- 

 face of the horizontal fibrous membrane (sf), and are 



attached by a special sinew to each side of the basal 



" See GCntheb, Aim., Mag. Nat. Hist., ser. 4, vol. VIII (1871), p. .S20, note. 



' In KSlheutkr's larva two. 



' According to LUtken's opinion of K6lreuter's larvie. Still we shonld observe that the larv;e of Diodon correspond iust as closely 

 to Richardson's larvae, especially in the position of the large spines; but iu the former the end of the notochord projects between the dorsal 

 and anal 6n8. A larva of this description, 2 mm. long and of ellipsoidal form, which was taken at the surface "between Java and the Cape of 

 Good Hope," has been obtained by the Royal Museum through Captain PflHL of Hamburg. In the same jar (and therefore taken probably at 

 the same time) lay another larva, 4 mm. long and of more distinct Diodon form, but still, like the preceding one, with the dorsal and anal 

 fins and the tip of the tail between them all situated on the ventral side. 



'' Cf. Wahlgren, Nagra anteckningar om en stor klumpjisk, Lunds Univ. Arsskrift. torn. IV. 



