THE SEA-SCORPION FAMILY. 139 



the latter, the head, and the yolk-sac. The marginal fin is 

 still continuous, and a few pigment-spots are dotted over it and 

 upon the tail. 



The next stage, about •235 inch long, is represented by 

 specimens caught in the sea on August 31st, which clearly 

 belong to the same series. The head, with eyes and brain, is 

 large, and the scoop-shaped indented snout is prominent. The 

 yolk-sac has nearly disappeared. The pectoral fins have still 

 further increased and the tail shows signs of becoming 

 heterocercal by the greater development of the lower rays. 

 By this time the little fishes have, even though so small, left 

 the surface-water and taken up the habitat of their older 

 brethren, namely the still water of the open sea at a depth of 

 about 25 fathoms. 



With a further growth in length, the head and snout 

 become proportionately larger and longer, and the little gurnard 

 is protected by the development of a number of conspicuous 

 spinous processes which make it a prickly morsel for predaceous 

 enemies. Two spines on the back of the head first appear, and 

 later, others are seen upon the operculum, the angle of the 

 lower jaw and the facial region. 



In the large pectorals there are thirteen fin-rays all joined 

 by membranes, but as the fish progresses in size the three most 

 ventral rays separate from the others and eventually become 

 free, both from the pectoral fin and from each other. They 

 form the three pairs of moveable ' feelers ' which are a well- 

 known feature of the adult gurnard, and which have been 

 carefully investigated at St Andrews by Mr H. C. Williamson". 

 This is a good instance of the repetition, in the development 

 of the individual, of a modification in structure which the 

 species has adopted in the course of time. Thus it is believed 

 that the gurnard is descended from ancestors which had all 

 the fin-rays united together as in most fishes, but that the three 

 ventral rays, being gradually specialised as feeling organs, 

 became separated from the rest until at last the free condition 

 of these rays, in the gurnards of the present day, was reached. 

 Hence we find that the young gurnards pass through the 



1 Vide 12th Keport Fishery Board for Scotland, p. 322, Plates 13 — 15. 



