91 



in Fig. 2 is that of a larva two days after hatching ; the larva from which the figure 

 was taken was 2*52 mm. in length. 



The eggs of the other species of Solea I have not yet seen, but for the sake of 

 comparison I have illustrated the development of several species of flat-fish. Figs. 

 3, 4, 5, Plate XVII, and Fig. 1, Plate XVIII, represent a series of stages in the 

 development of the common flounder, Pleuronectes flesus, from the time of hatching 

 till the completion of the metamorphosis. Fig. 3, Plate XVII, is from a larva hatched 

 under artificial conditions on February 18, 1889, and drawn two days afterwards : 

 its length was 3'56 mm. The chromatophores are arranged in such a way as to form 

 two transverse bands, one at the level of the anus, the other some distance behind it ; 

 two dark bands in these positions are more or less conspicuous in several species of 

 Pleuronectes in the adult condition. Fig. 4 shows the same larva when the yolk has 

 been entirely absorbed and the mouth and gill-slits have been developed : its length was 

 3' 94 mm. The pectoral fin is relatively very large at this stage. This condition was 

 reached in confinement on February 24, by larvae hatched on February 18. The next 

 stage was observed in young fish found in the harbour at Mevagissey at low tide on 

 April 2, and is represented in Fig. 5. The length of the fish from which this 

 figure was drawn was 10*5 mm., or a little over three-eighths of an inch. This stage 



O 7 O O 



is the beginning of the metamorphosis. The left eye has travelled upwards so as to 

 project above the edge of the head, but it is still on the left side. The little fish in 

 this condition swims on its edge, but slightly inclined to the left side. It is still 

 extremely transparent, so that when alive it is only rendered visible by the metallic 

 brilliancy of the choroid coat of the eyes, which shine through the transparent tissues 

 in the sun like two metal beads. The yellow spot in the visceral cavity in the figure 

 is the gall bladder. Fig. 1, Plate XVIII, shows a later stage in which the left eye 

 has reached the edge of the head, so that its lens and cornea are visible on the right 

 side. The fish from which this figure was taken was actually smaller than that 

 represented in Fig-. 5, Plate XVII, probably because its metamorphosis had begun 

 somewhat earlier. It seems that in some individuals the metamorphosis is completed 

 while they are smaller in size than others at an earlier stage of development. The 

 fish represented in Fig. 1, Plate XVIII, was exactly 1 cm. in length. It was much 

 more opaque and more pigmented than the stage previously described ; in fact, though 

 slightly translucent when seen under the microscope by transmitted light, to the 

 unaided eye it appeared quite opaque. It will be noticed that the interval of time 

 between the stage of Fig. 4, and that of Fig. 5, Plate XVII, was about five weeks, and 

 that in that time nearly all the organs had reached their final form. The muscles 

 and skeleton have developed very greatly. The part of the body behind the anus 

 has increased very much in dorso-ventral breadth, and in it the vertebrae with their 

 spines, the interspinous bones, and the fin rays have been completely formed. 



Fig. 2, Plate XVIII, represents the larva of the dab (Pleuronectes limandd] imme- 

 diately after hatching. The larva from which this figure was taken was hatched in 



x 2 



