The Origin of Vertebrates

by Walter Holbrook Gaskell

It is different, however, when an attempt is made to connect two of the main divisions themselves. It is true enough that there is every reason to believe that the arthropod group has been evolved from the segmented annelid, and so the whole of the segmented invertebrates may be looked on as forming one big division, the Appendiculata, all the members of which will some day be arranged in orderly sequence, but the same feeling of certainty does not exist in other cases.

In the very case of the origin of the Appendiculata we are confronted with one of the large problems of evolution—the origin of segmented from non-segmented animals—the solution of which is not yet known.

Theories of the Origin of Vertebrates.

The other large problem, perhaps the most important of all, is the question of the relationship of the great kingdom of the Vertebrata: from what invertebrate group did the vertebrate arise?

The great difficulty which presents itself in attempting a solution of this question is not so much, as used to be thought, the difficulty of deriving a group of animals possessing an internal bony and cartilaginous skeleton from a group possessing an external skeleton of a calcareous or chitinous nature, but rather the difficulty caused by the fundamental difference of arrangement of the important internal organs, especially the relative positions of the central nervous system and the digestive tube.

Fig. 1.—Arrangement of Organs in the Vertebrate (A) and Arthropod (B).

Al, gut; H, heart; C.N.S., central nervous system; V, ventral side; D, dorsal side.

Now, if we take a broad and comprehensive view of the invertebrate kingdom, without arguing out each separate case, we find that it bears strongly the stamp of a general plan of evolution derived from a cœlenterate animal, whose central nervous system formed a ring surrounding the mouth. Then when the radial symmetry was given up, and an elongated, bilateral, segmented form evolved, the central nervous system also became elongated and segmented, but, owing to its derivation from an oral ring, it still surrounded the mouth-tube, or œsophagus, and thus in its highest forms is divided into supra-œsophageal and infra-œsophageal nervous masses. These latter nervous masses are of necessity ventral to the digestive tube, because the mouth of the cœlenterate is on the ventral side. The striking characteristic, then, of the invertebrate kingdom is the situation of a large portion of the central nervous system ventrally to the alimentary canal and the piercing of the nervous system by a tube—the œsophagus—leading from the mouth to the alimentary canal. The equally striking characteristic of the vertebrate is the dorsal position of the central nervous system and the ventral position of the alimentary canal combined with the absence of any piercing of the central nervous system by the œsophagus.

So fundamentally different is the arrangement of the important organs in the two groups that it might well give rise to a feeling of despair of ever hoping to solve the problem of the Origin of Vertebrates; and, to my mind, this is the prevalent feeling among morphologists at the present time. Two attempts at solution have been made. The one is associated with the name of Geoffrey St. Hilaire, and is based on the supposition that the vertebrate has arisen from the invertebrate by turning over on its back, swimming in this position, and so gradually converting an originally dorsal surface into a ventral one, and vice versâ; at the same time, a new mouth is supposed to have been formed on the new ventral side, which opened directly into the alimentary canal, while the old mouth, which had now become dorsal, was obliterated.