THE POPULAR SCIENCE MONTHLY.
the stomach, intestine, heart, and other organs of vegetative life. This is shown in Fig. 3.
Let us now go one step further and learn what kind of a vertebrate is the gar-pike. At present the most natural primary subdivision of the branch seems to be into three great groups. The highest
Fig. 2.—Part of the Side op the Body of Lepidosteus platystomus,
Natural size, showing the arrangement of the enameled scales. Below is an outline of a single scale; the point is covered by the scale in front.
is the Mammalia, comprising our common quadrupeds, also bats, monkeys and men, seals and whales. The females of all these bring forth their young alive, and nourish them with milk.
Next come the Sauropsida, including birds, turtles, crocodiles, lizards, and snakes. Lastly, the Ichthyopsida, embracing the Batrachians (frogs, toads, and salamanders), and all other vertebrates.
Evidently, our gar-pike is neither a mammal nor a bird, a turtle, a snake, nor a lizard. It does look a little like an alligator, but it has not only fins and scales, but also gills, which are not known to exist in any reptile; while all the Ichthyopsida have gills during at least a part of their lives. The gar-pike is neither a frog nor a toad; it has scales and fin-rays unlike salamanders. Why, then, not call it a fish?
Fig. 3.—Cross-Section (Natural Size) of the Short-nosed gar-pike (Lepidosteus platystomus),
Showing the general arrangement of the organs which is characteristic of vertebrates. The section is made in front of the ventral fins at the point indicated by X on Fig. 1. The cut surface is looked at from behind. Near the middle is the vertebral column or backbone (VC). Above it is the spinal cord (SC), surrounded by bony walls. Below are the abdominal viscera and A is the median aorta. V V the lateral veins. MA is the median channel of the air-bladder, and LA, LA, are its lateral chambers. The cavity of the stomach (Al) is on the left, and the liver (L). with two veins, on the right. O, O are the two ovaries, of which the left lies farther forward so that its section is smaller. The whole is surrounded by the muscular walls of the body (M, M, M, M), and this again is covered by the plates of the skin.