The eggs of fishes vary very much in size and form. In those sharks and rays which lay eggs the ova are deposited in a horny egg-case, in color and texture suggesting the kelp in which they are laid. The eggs of the bullhead sharks (_Heterodontus_) are spirally twisted, those of the cat-sharks (_Scyliorhinidæ_) are quadrate with long filaments at the angles. Those of rays are wheelbarrow-shaped with four "handles." One egg-case of a ray may sometimes contain several eggs and develop several young. The eggs of lancelets are small, but those of the hagfishes are large, ovate, with fibres at each side, each with a triple hook at tip. The chimæra has also large egg-cases, oblong in form. [Illustration: FIG. 95.--Egg of the Hagfish, _Myxine limosa_ Girard, showing threads for attachment. (After Dean.)] In the higher fishes the eggs are spherical, large or small according to the species, and varying in the firmness of their outer walls. All contain food-yolk from which the embryo in its earlier stages is fed. The eggs of the eel (_Anguilla_) are microscopic. According to Günther 25,000 eggs have been counted in the herring, 155,000 in the lumpfish, 3,500,000 in the halibut, 635,200 in the sturgeon, and 9,344,000 in the cod. Smaller numbers are found in fishes with large ova. The red salmon has about 3500 eggs, the king salmon about 5200. Where an oviduct is present the eggs are often poured out in glutinous masses, as in the bass. When, as in the salmon, there is no oviduct, the eggs lie separate and do not cohere together. It is only with the latter class of fishes, those in which the eggs remain distinct, that artificial impregnation and hatching is practicable. In this regard the value of the salmon and trout is predominant. In some fishes, especially those of elongate form, as the needle-fish (_Tylosurus_), the ovary of but one side is developed. [Illustration: FIG. 96.--Egg of Port Jackson Shark, _Heterodontus philippi_ (Lacépède). (After Parker and Haswell.)] =Protection of the Young.=--In most fishes the parents take no care of their eggs or young. In some catfishes (_Platystacus_) the eggs adhere to the under surface of the female. In a kind of pipefish (_Solenostomus_), a large pouch for retention of the eggs is formed on the belly of the female. In the sea-horses and pipefishes a pouch is formed in the skin, usually underneath the tail of the male. Into this the eggs are thrust, and here the young fishes hatch out, remaining until large enough to take care of themselves. In certain sea catfishes (_Galeichthys, Conorhynchos_) the male carries the eggs in his mouth, thus protecting them from the attacks of other fishes. In numerous cases the male constructs a rough nest, which he defends against all intruders, against the female as well as against outside enemies. The nest-building habit is especially developed in the sticklebacks (_Gasterosteidæ_), a group in which the male fish, though a pygmy in size, is very fierce in disposition.