LARVAL AND POST-LARVAL FISHES—REGAN. 151 
out into filaments which may be used as feelers, perhaps to find food at night. 
When the pelvic fins develop precociously they also may take the form of spines 
(e.g., Thyrsites) or of long filaments (e.g., Trachypterus) ; but in other cases they grow 
out into large fins with the rays fully connected by membrane (e.g., Pagetopsis 
macropterus, p. 132, Pl. III, figs. 1-3); presumably such fins would be used to help the 
pectorals in balancing and to prevent the fish from sinking. 
It seems that in most fishes the fins develop in the same order and that the 
development of one or more fins out of their turn, precociously, may be for purposes 
either of flotation, balance, defence, or perhaps nocturnal feeding. 
4. CHARACTERISTIC FEATURES OF PELAGIC LARVAE. 
Absence of accessory organs of respiration and of adhesive organs.—Larval 
structures that occur in more than one group of fresh-water fishes are external gills 
and adhesive organs; the latter enable them to remain in the place selected by the 
parents until the yolk is absorbed, and external gills are advantageous when the water 
is limited in quantity or deficient in oxygen; that these structures are not found in 
pelagic marine larvae is not surprising. 
Tnvisibility.—All pelagic larvae are transparent, and in some groups the larval 
stage is prolonged until a considerable size is reached; these large larvae remain 
transparent owing to the strong compression of their bodies and the looseness of their 
tissues, and the change into the less compressed and more compact young fish is 
accompanied by a shrinkage. This type of development is characteristic of the Apodes 
(p. 140, Pl. VII, figs. 5-7), but it occurs also in the Elopidae and Albulidae. 
Buoyancy and balance—It has been suggested that the dorsal sinus of the 
Myctophidae (pp. 127, 138, Pl. VI) may serve as a float, and it seems likely that the 
large pelvic fins of the larval Chaenichthyidae, especially Pagetopsis (p. 132, Pl. III, 
figs. 1-3), may be spread to prevent the fish sinking, 
Generally the vertical fin acts as a keel, and balance is maintained by movements 
of the pectorals; possibly the protrusion of the terminal part of the gut in many 
larvae may be connected with balance; in those described as Stylophthalmus the 
protruded portion may be quite long, but it is difficult to understand why this feature 
should be so exaggerated as it is in Stylophthalmus macrenteron (p. 136, Pl. V, fig. 1). 
The terminal part of the gut, with its basal support, forms a long appendage; this 
would, presumably, keep the fish steady, and make it difficult for it to turn over on its 
back, but would retard its progress if it attempted to swim; in fact, one may suppose 
that the chief effect of flexions of the tail would be to rotate the fish, the anal 
appendage serving as a fulcrum. 
Sense organs.—Certain fin-rays may be formed precociously and grow out into 
long filaments; these may be used as feelers, perhaps to find food at night. In all 
larvae the auditory, optic and olfactory organs appear to be well developed, but it is 
y 2 
