90 



similarity of the young sole at tliis minute size to the full-grown adult is evident from 

 the figure. The chief difference is in the relations of the intestine. The right lateral 

 diverticulum of the body cavity is scarcely at all developed, and the four lengths of 

 intestine which occupy that diverticulum in the adult are absent. The intestine 

 extends backwards only very slightly beyond the anterior ventral interspinous bone. 

 The dorsal eye is slightly nearer to the edge of the head than in the adult, otherwise 

 the metamoi-phosis is complete. The colour has disappeared from the lower side, the 

 markings of the species are completely developed on the upper side. The pigmentation 

 of the upper side is not nearly so dense as in the adult, and the whole body is some- 

 what translucent, but nevertheless when the young fish is seen in the living state by 

 reflected light, whether resting on the bottom or swimming horizontally in the water, 

 its upper side shows the same colour as the adult, and undergoes the same changes of 

 colour on different materials in consequence of the action of light. The identification of 

 the young sole, Solea vulgaris, at this stage is neither doubtful nor difficult, for, although 

 some of the characters of the adult are not discernible, others can be perceived easily 

 enough. The large number of anal fin rays distinguishes it from either S. variegata 

 or viinuta, while the tubular form of the anterior left nostril distinguishes it from 

 lascaris. I obtained specimens of this stage on three occasions within a short time, in 

 1889, from the shore at low water at spring tides. On the third occasion I received 

 only one specimen which was nearly three-quarters of an inch long (18 mm.). 



I could not trace the further development of these young soles, for they disappeared 

 from the shore. But no important changes of structure were required to produce the 

 adult, further development consisted almost entirely in the increase of size. For the 

 discussion of the later growth reference must be made to the next part of this 

 memoir. 



The only other species of Solea whose development I have been able to study is 

 S. variegata. The eggs of this species are smaller than those of vulgaris, measuring 

 1-28 to 1-36 mm. in diameter. The egg, Plate XVI, Fig. 6, is easily recognised and 

 distinguished from that of S. vulgaris by the peculiarity of its oil globules, which, instead 

 of being very minute and numerous and aggregated in a number of distinct groups, 

 are of considerable size and scattered singlj' and separately all over the surface of the 

 yolk. The external layer of segmented yolk is present as in S. vulgaris. The develop- 

 ment of course takes place in the same way as in the latter. "When the chromatophores 

 appear they form another distinguishing feature ; there are both yellow and black 

 chromatophores as in vulgaris, but the foriner are much lighter in this species, inclining 

 to lemon colour, while those of vulgaris are darker. 



Figs. 1 and 2 on Plate XVII show two stages of the hatched larva of the " thick- 

 back." The younger. Fig. 1, immediately after hatching, has of course the largest 

 yolk-sac and the shortest length of body; it is 2-42 mm. in length. The heart 

 is just visible, but is compressed between the yolk and the under side of the throat, 

 the pericardium being but slightly developed and scarcely visible. The stage shown 



