The Lancelet also differs from the true vertebrates, in that it has no limbs. There is a fringing fin along the body, but it is not comparable with the fins of fishes. It differs also in possessing no teeth.
In one respect, however, the Lancelet reminds us of a fish: and that is in the arrangement of its muscles; these form a successive series of overlapping masses on each side of the body, as in a fish.
The development of the Lancelet presents us with an instance of the two-layered larva, or Gastrula. This shows that Amphioxus is a comparatively primitive type. But it has been suspected that it is less primitive than it looks, and that it has degenerated from some higher form, owing to its preferring a dull mode of existence, half-buried in sand or mud.
There is a huge gap between the Lancelet and the true vertebrates. The lowest form of the latter is Ammocœtes, the larva of the Lamprey (Petromyzon). The latter, even in the adult form, has no true limbs, though there are fringing fins. The notochord sheath is supplemented, however, by cartilage bars which are equivalent to the beginnings of the vertebræ of the back-bone. The gills are very different from those of other true vertebrates, and it has no jaws. Teeth it has, however, on the tongue and the lining of the mouth. Probably this creature is greatly altered by adaptation to its peculiar mode of life, so that no certain conclusions can be drawn from it regarding the structure of primitive fishes. It has a sucking mouth, by means of which it hangs on to fishes, while it rasps away their flesh with its rough tongue. When not thus engaged, it hangs on to a stone by means of its suctional mouth, thus fixing itself at rest. The Hag-fish, Myxine, in many respects similar, devours dead fishes chiefly. The Hag-fish is found on English coasts: so is the Marine Lamprey; while two freshwater forms are found in streams.
Leaving the Cyclostomata, as the above fishes are called, we reach the true fishes, which have limbs and scales. Something has already been said regarding their teeth and gills. The Cartilaginous fishes, in which most part of the skeleton remains gristle and does not become transformed into bone, include the Sharks, Rays, and Dog-fishes, all savage animals with strong teeth. The common spotted Dog-fish of our own shores is familiar to everybody: fishermen regard it with disgust, as it is not eatable. The Rays are flattened fishes, which live at the bottom of rather deep water, and attain enormous size even on our own coasts. The Thornback Skate is covered with prickles (placoid scales). All these fishes are grouped under the name of Elasmobranchii, the Strap-gilled, so called from the structure of the gill-arches.
The majority of familiar fishes, such as the herring, mackerel, cod and sole, belong to the group of Teleostei, or Bony Fishes, in which, by contradistinction from the last group, as much of the skeleton as possible becomes bone. Nevertheless, traces of the notochord persist in the back-bone of these fishes. Break the back-bone across, of a cod or a sole, and you will find between adjacent sides of the centra, or middle parts of the vertebræ, a pad of gristly substance. This is the remaining substance of the notochord, which finds room between the cup-shaped sides of the centra. When the centrum, instead of being biconcave, is solid, as in the higher Vertebrata, the notochord is obliterated by its encroachment.
The Amphibia, familiarly represented by Frogs and Toads, receive their name, "adapted for both lives," from the fact that they usually divide their lives between land and water. They are, from one point of view, the most interesting of the classes of the Vertebrata, for they form a dividing line between the lower and upper Chordata. Below we have Hemichordata, Ascidians, Amphioxus, Fishes; all water-dwellers, breathing by gills. Above, we have Reptiles, Birds, Mammals, air-breathers, never possessing gills, except for a short time, as rudiments in the embryo, not brought into use. They are linked by the Amphibia, in which we see the larva a water-dweller, breathing by gills; the adult, an air-breather, adapted for life on land, and obliged to come to the surface to breathe, even when it passes its time in the water. The individual Amphibian tells us the past history of the higher groups; once they had gills—but growing older, they lost them.
[Fig. 43] shows us an outline sketch of Amphibian larvæ; we should require an enlarged diagram of an earlier stage, to show the gills, which are external and projecting at first, but afterwards are overgrown by the skin with the exception of an orifice on each side. The diagram shows the gradual change of form. The tails in these tadpoles will presently be lost, for they belong to the Anura, or tail-less order of Amphibia (Frogs and Toads). The tailed Amphibians, Urodela, are represented in Great Britain by the Newts, Triton, popularly called Efts. Belonging to the Tailed Amphibians also, is the Axolotl, a creature found in the lakes of Mexico, and in those of the Rocky Mountains. It may or may not retain its gills; and forms with gills, and forms without, may be found in the same lake, each capable of laying eggs. The two forms were at first described under two different generic names: but when specimens of the gill-bearing Siredon, kept in confinement, lost their gills, it was seen that they became Amblystoma. There are other cases of larval forms that produce young, and this curious occurrence is known as "pædogenesis."
