114 
SPOLIA ZEYLANICA. 
arises, is much more highly pigmented than the dorsal portion ; 
neither of them shows any trace of secondary rays. The fascicula- 
tion of the primary caudal rays (i.e., the striations), which was 
foreshadowed on the fifteenth day in some individuals, has made 
further progress, but I noted that the developing secondary caudal 
rays were not yet provided with the basal prongs by which they 
subsequently become articulated with the cartilages, nor had they 
become transversely jointed. 
Larvae preserved on the twenty-eighth day after hatching varied in 
length from 8*5 to 10 mm., with ten to twelve caudal rays, of which 
the larger are jointed and provided with a pair of basal prongs astride 
of the basal cartilages (Fig. 13). The caudal fin-rays all lie below 
the upturned end of the notochord ; they consist of two groups, 
upper and lower, separated by a special notch in the basal cartilage, 
through which the blood is transmitted to the caudal capillary 
system (Fig. 13). The fin-rays are controlled by special divaricator 
muscles (not shown in the figure), which arise from the skeletogenous 
tissue below the notochord at its point of flexure, and can 
be traced to the ends of the radial prongs, the fibres which supply 
the upper group passing obliquely upwards parallel to the 
notochord. 
Larvae thirty-seven to forty days old vary in length from 10 to 
13 mm. The length bears no direct proportion to the bulk, since a 
larva of 13 mm. has at least twice the bulk of one of 10 mm. This is 
the transition period from the larval to the postlarval phase of growth. 
At 10 mm. there are still no external rudiments of the ventral fins, but 
these appear when the larva has attained the length of 10 • 25 mm. 
They arise in situ a short distance behind the plane of the pectoral 
fins and far in front of the vent, as minute buds close to the middle 
line of the abdominal surface. The embryonic fins are still continu¬ 
ous, but a shallow constriction, both above and below, separates 
the future definitive dorsal and anal fins from the caudal fin, the 
intervening portions of the embryonic fin in the region of the caudal 
peduncle undergoing reduction and degeneration, accompanied by 
the appearance of vacuoles in their substance (Fig. 14). The 
primordial formative tissue of the dorsal and anal rays has now 
invaded the corresponding parts of the embryonic fin, obscuring the 
striations, leaving a peripheral rim free, where the primary striations 
are clearly visible. The anal rays appear in the middle of the 
substance of the fin, separated by an interval from the basal line 
and surrounded by dense pigment. The dorsal rays arise from the 
basal line (Fig. 14), and the pigment is sparse. 
The larvae are now swimming near the bottom of the shallow 
aquarium in which they have been reared and come to the surface 
to take in air. On the twenty-eighth day a larva gulped the air once 
a minute, eight times in eight minutes, each time leaving a small 
air-bubble at the surface. 
