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ART. XXIV.-On a Nero Species of Stickleback (Gasterosteus gymnetes). By J. W. Dawson, LL.D., &c.
(Read before the Natural History Society.) I propose in this paper to redeem from unmerited neglect an inhabitant of our brooks, the six-spined stickleback, wbieb, though very well known to the boys of Montreal, and much persecuted by them, has, in so far as I am aware, hitherto escaped the notice of naturalists. It belongs to a group of little fishes, represented by many species in the fresh-waters and on the coasts both of the old and new world, and remarkable for their armature of sharp spines, their active and pugnacious habits, and the provision which they make for the care of their spawn.
Fig. 1.--Gasterosteus gymnetes (male). "The present species makes its appearance in the brooks immediately after the melting of the snows in spring. It is then plump and active, and the females are laden with spawn. The spawn is deposited in the end of March or beginning of April, in a globular OUXADIAN NAT.
VOL. IV. No. 5
nest about the size of a musket-bullet, constructed of green alga, and placed in a tuft of submerged grass or aquatic weeds. My eldest boy, who first showed me the nest, assures me that one of the parents, probably the male, as in the case of a common British species, remains near the precious deposit, and drives away all intruders. The ova are translucent and colourless, and of the size of a pin-head. They soon exhibit to a close inspection with the naked eye or a magnifying-glass of moderate power, two black specks, the rudiments of the eyes of the future fish ; and under the microscope present the appearance represented in figure 2,
Fig. 2.-Egg of G. gymnetes (magnified). the embryo being coiled up in the usual manner around the yolkbag, and occasionally moving by convulsive jerks. At this stage I observed that microscopic animalcules had obtained access to the interior of several of the eggs, and evidently occasioned annoyance to the embryo. I have reason to believe that several embryos were destroyed in this way, and perhaps the carefully-built nest may have for one of its objects to guard against such attacks.
In two or three weeks the young extricate themselves from the egg—still only about a tenth of an inch in length, and having the yolk bag attached to the abdomen, as represented in figure 3.
Fig. 3.—Embryo of G. gymnetes (magnified). They swim quickly, and are nearly as dexterous as the adults in avoiding danger and availing themselves of places of concealment. They are now very beautiful objects for microscopic investigation. The head appears a rounded mass of cells. The eyes, however, are well developed, and can be rotated as perfectly as in the adult.
The pulsation of the heart, seen immediately in front of the yolkbag, and the movement of the blood in the vessels of the yolkbag, in the aorta and the large veins above and below the spinal column, as well as in the transverse branches passing between the muscular flakes of the sides, can be very distinctly perceived. The spinal column appears as a uniform cartilaginous hyaline cord, and the pectoral fins are fully developed and in constant motion. The posterior part of the body is surrounded by a delicate membranous fin, terminating in a rounded point at the tail.
As the yolk-bag diminishes in size, beautiful stellate pigment cells become developed in the skin, and render it too opaque to permit the interior structures to be well seen; and before the little creature has attained the length of one-sixth of an inch, it has all the appearance of the adult, and may be seen slowly swimming or hovering, with its bright eyes rolling in search of the minute crustaceans, worms, rotifers and animalcules which form its prey, and which it seizes by sudden, quick darts. When alarmed, it hides under stones or algæ, or remains motionless over some part of the bottom resembling its own colour, which, when it is irritated or frightened, deepens almost into black.
The fry remain in the brooks throughout the spring and early summer; but the greater part disappear, descending I suppose into the river, before autumn. Those that remain are now (September) nearly an inch in length, and will probably be fullgrown next spring.
The following is the description of the adult in spring :
Length, two inches; head to body, as 1 to 4; depth of body to length, as 1 to 4. Form, compressed, especially above; back, regularly arched to the end of the dorsal fin, from which it curves upward slightly to the insertion of the caudal fin. Liead, regularly conical, obliquely truncated by the lower jaw. Eye, prominent, diameter one-tenth of an inch. Nostril, half-way between eye and pre-maxillary, and on a level with the upper part of the orbit. Body, quite destitute of bony plates; on this last character, the absence of defensive armour, I have given it the specific name gymnetes. Pre-operculum, bent nearly at a right angle, rounded at apex, Operculum, rounded below, nearly straight above, rounded at superior posterior angle. Brancheostegal rays, three. Mucous pores, three above each eye, a few very small under the eye; on the occiput a curved row of pores convex backward ; at the edge of the operculum two less distinct rows convex upward.