134 Mr. D. M. Paul on the Pelvic Fins in the 



betakes itself to the rock-pools and its bluish-green back and 

 silvery sides give place to the uniform brown coloration of 

 the adult. 



The fin may now be described at several stages in more 

 detail. In the earliest stage obtained tlie fish measured 

 slightly less than 5 millim. The fin (of which PI. V. fig. 1 is 

 a drawing) extends a considerable distance behind the anus ; 

 it measures 1'8 millim., ?'. e. rather more than one third of 

 tlie total length of the fish. The fin-rays are four in number 

 and are of equal length. At the tips they tend to become 

 split up into a number of fine filaments. The pigment in 

 this particular specimen was small in amount and confined to 

 the tip of the fin, the remainder being yellowish. At this 

 stage the pectoral fin is about one third the size of tlie pelvic 

 and is fan-shnped. 'J'he base of the pelvic fin is only very 

 slightly in front of the pectoral at this stage, thereby con- 

 trasting with its situation in the adult, where it is considerably 

 in front. This shows that, as in other species, the tendency 

 of the pelvic fins is to shift forward. The ejjithelium, as 

 shown in transverse section, is thin and undifferentiated; it 

 is not wrinkled between the fin-rays, as in somewhat older 

 specimens, showing that the fin caimot be spread in the form 

 of a fan, as in later stages. The pigment varies greatly in 

 different fins at the same stage; this may be due to individual 

 variation, but more probably because all the specimens 

 examined had been preserved in spirit, some for a considerable 

 number of years. As the young fish grows the pelvic fins 

 increase in length, but still keep the same form. The black 

 pigment becomes more and more confined to the tip of the 

 fin, so that when the fish is 10-12 millim. long the extreme 

 tip of the fin alone is deep black, while the rest is pale in 

 colour. At this stage the fin measures about one fourth of 

 the total length. 



Fig. 2 (PI. V.) represents the fin of a young rockling which 

 was still pelagic, although it had grown to a size of 20 millim. 

 The tips of the fin-rays are not now split, thougii they are 

 still very long. The number of fin-rays has increased to 

 six, and of these the second is slightly the longer, while the 

 fifth and sixth are short. Sections near the base of the fin 

 at this stage show tl:at it differs from the adult only by the 

 less highly differentiated epithelium. Between the fin-rays 

 the epitiielium is now much folded, so that the fin can be 

 spread out like a fan. This is connected with the fact that 

 tiie fish leads a pelagic life and seeks its food near the surface, 

 the fin being used mainly, if not wholly, as a locomotor 

 organ, while in adult life it is used chiefly as a tactile organ. 



