Postlarval and Young Stages o/Onus mustela. 133 



The present paper is an attempt to give a somewhat more 

 detailed account of this latter part of the subject. 



In their young stages many fishes have greatly developed 

 paired fins, while in the adult these are less in proportion. 

 In some cases it is the pectoral fins which are thus hyper- 

 troi)hied, while in other cases it is the pelvic fins. The 

 young gurnards, for example — both the red and grey species 

 — have large pectoral fins, while in the ling the young has 

 huge pelvic fins which become reduced in proportion as the 

 fish grows. In the various species of rockling it is always 

 the pelvic fins which are thus enlarged. The following table 

 gives the length of the body and that of the pelvic fin at 

 various stages ; the figures are only what may be called the 

 mean measurements, different specimens varying greatly in 

 the development of the fins. The measurements are all 

 expressed in millimetres, and apply only to spirit-specimens. 



As has been already stated, this development is subject to 

 considerable variation : for example, of two specimens each 

 measuring ll*o millim. the pelvic fins measured 2-8 and 

 3T millim. That with the longer fin was captured in 

 August and that with the shorter fin in May; but as the 

 rockling spawns from March to December this proves nothing. 

 The condition of the other fins, however, points to the conclu- 

 sion that the fish with the longer fin was slightly older than 

 the other. The table indicates that the growth of the pel vies 

 is at first extremely rapid, but a point is reached at which an 

 actual reduction in size of the fin takes place. Like all the 

 rest of the development, this feature is extremely variable, 

 but the mean may be taken to lie between 20 and 25 millim., 

 although we may get specimens of 17 millim. whose fins have 

 undoubtedly passed this critical point. Several specimens 

 over 25 millim. have been examined whose fins show no signs 

 of reduction. About this time a change comes over the 

 habits of the fish. It has hitherto been pelagic, but now it 



