ON SOME LARVAL STAGES OP PISHES. 69 



hatched larva the intestine is straight, and lies in a depression of 

 the dorsal surface of the yolk, while in the present stage the intes- 

 tine shows the commencement of a convolution above the yolk, and 

 the latter does not project dorsalwards on each side of the gut. 

 The whole of the anterior part of the body is now straightened out, 

 while at the time of hatching it was bent downwards towards the 

 anterior extremity. Accordingly the region of the fore-brain and 

 olfactory organ is now at the anterior extremity of the axis of the 

 body. The position of the olfactory organ is indicated by ol. in the 

 figure, although the organ itself was not distinct enough to be 

 represented in the figure when the drawing was made. The intestine 

 or alimentary tube has now grown forwards beneath the brain to 

 meet the skin beneath the fore-brain and eyes, and the lumen or 

 cavity of the tube, open in the posterior part, is faintly indicated 

 anteriorly by a line ; the mouth, however, is not yet open, although 

 the lumen of the gut extends to the exterior at the anus. The 

 heart, ht., has the usual structure in larval fish at this stage ; it 

 consists of a tube beneath the gullet having an aperture posteriorly, 

 by which it communicates with the perivitelline sinus. The heart 

 is surrounded by a cavity, which is separated from the perivitelline 

 sinus by a delicate membrane, and which is afterwards extended to 

 form the body-cavity. Pigment has begun to appear in the eye, 

 i. e. in the choroid membrane, as separate dots. The mid-brain, 

 m. b., projects dorsally. The primordial membranous fin of the 

 larval sole is characterised by its irregularity of outline anteriorly. 

 The edge of this fin forms a projection over the mid-brain, and 

 another behind the head. The pigmentation of the skin consists of 

 black and yellow dendritic {i. e. branched) chromatophores. On 

 the fin these chromatophores form four not very definitely marked 

 spots on the dorsal portion, and one posteriorly on the ventral 

 portion. At later stages these spots become more definite and 

 conspicuous. The total length of the larva represented in fig. 1 

 was 4'35 mm. 



Fig. 2 shows an older larva from the same lot. It was drawn on 

 April 28th, that is, six days after hatching. The mouth is now 

 fully developed, although a remnant of the yolk is still unabsorbed, 

 and the larva has not yet begun to feed. The eye is now com- 

 pletely pigmented, the choroid membrane being black and opaque. 

 The auditory vesicle, in the previous stage a simple spherical 

 capsule with two calcareous nodules inside it, is now much enlarged, 

 and exhibits the commencement of the semicircular canals within 

 it. The olfactory organ, ol., has the form of a spherical capsule 

 just beneath the skin, opening to the exterior by a circular aperture. 

 There is, of course, one of these on each side of the snout ; the 



