Science shows the direct relationship between the Vertebrates, and the Invertebrates by means of several connecting-links, the most noticeable of which is the Lancelot, a creature resembling the fish-form, and yet also closely resembling the lower (invertebrate) forms of life. This creature has no head, and but one eye. It is semi-transparent, and possesses cilia for forcing in the water containing its food. It has something like gills, and a gullet like the lower forms. It has no heart, the blood being circulated by means of contracting vessels or parts. Strictly speaking, it has no back-bone, or vertebra, but still Science has been compelled to class it among the vertebrates because is has a gristly cartilage where the back-bone is found in the higher forms. This gristle may be called an “elementary spine.” It has a nervous system consisting of a single cord which spreads into a broadened end near the creature’s mouth, and which may therefore be regarded as “something like a brain.” This creature is really a developed form of Invertebrate, shaped like a Vertebrate, and showing signs of a rudimentary spine and nervous system of the latter. It is a “connecting-link.”
The lowest forms of the true Vertebrates are the great families of Fishes. These Fish families include fishes of high and low degree, some of the higher forms being as different from the lowest as they (the highest) are different from the Reptile family. It is not necessary to go into detail regarding the nature of the fish families, for every student is more or less familiar with them.
Some peculiar forms of fish show a shading into the Reptile family, in fact they seem to belong nearly as much to the latter as to their own general family. Some species of fish known as the Dipnoi or “double-breathers,” have a remarkable dual system of breathing. That is, they have gills for breathing while in the water, and also have a primitive or elementary “lung” in the shape of an air-bladder, or “sound,” which they use for breathing on land. The Mud-fish of South America, and also other forms in Australia and other places, have a modification of fins which are practically “limbs,” which they actually use for traveling on land from pond to pond. Some of these fish have been known to travel enormous distances in search of new pools of water, or new streams, having been driven from their original homes by droughts, or perhaps by instincts similar to the migrating instinct of birds. Eels are fish (although many commonly forget this fact) and many of their species are able to leave the water and travel on land from pond to pond, their breathing being performed by a peculiar modification of the gills. The climbing perch of India are able to live out of water, and have modified gills for breathing purposes, and modified fins for climbing and walking. So you see that without leaving the fish family proper, we have examples of land living creatures which are akin to “connecting links.”