The Origin of Vertebrates

by Walter Holbrook Gaskell

Closely bound up with the respiratory system is the nature of the circulation of blood through the gills. Before, therefore, proceeding to the consideration of the segments in front of those which carry branchiæ, it is worth while to compare the circulation of the blood in the gills of Limulus and of Ammocœtes respectively.

In all the higher vertebrates the blood circulates in a closed system of capillaries, which unite the arterial with the venous systems. In all the higher invertebrates this capillary system can hardly be said to exist; the blood is pumped from the arterial system into blood spaces or lacunæ, and thus comes into immediate contact with the tissues. From these it is collected into veins, and so returned to the heart. There is, in fact, no separate lymph-system in the higher invertebrates; the blood-system and lymph-system are not yet differentiated from each other. This also is the case in Ammocœtes; here, too, in many places the blood is poured into a lacunar space, and collected thence by the venous system; a capillary system is only in its commencement and a lymph-system does not yet exist. In this part of its vascular system Ammocœtes again resembles the higher invertebrates more than the higher vertebrates.

This resemblance is still more striking when the circulation in the respiratory organs of the two animals is compared. A branchial appendage is essentially an appendage whose vascular system is arranged for the special purpose of aerating blood. In the higher vertebrates such a purpose is attained by the pulmonary capillaries, in Limulus by the division of the posterior surface of the basal part of the appendage into thin lamellar plates, the interior of each of which is filled with blood. The two surfaces of each lamella are kept parallel to each other by means of fibrous or cellular strands forming little pillars at intervals, called by Macleod "colonettes." A precisely similar arrangement is found in the scorpion gill-lamella, as seen in Fig. [69], A, taken from Macleod. In Ammocœtes there are no well-defined branchial capillaries, but the blood circulates, as in the invertebrate gill, in a lamellar space; here, also, as Nestler has shown, the opposing walls of the gill-lamella are held in position by little pillar-like cells, as seen in Fig. [69], B, taken from his paper.

In this representative of the earliest vertebrates the method of manufacturing an efficient gill out of a lacunar blood-space is precisely the same as that which existed in Limulus and the scorpion, and, therefore, as that which existed in the dominant invertebrate group at the time when vertebrates first appeared. This similarity indicates a close resemblance between the circulatory systems of the two groups of animals, and therefore, to the superficial inquirer, would indicate an homology between the heart of the vertebrate and the heart of the higher invertebrate; but the former is situated ventrally to the gut and the nervous system, while the latter is composed of a long vessel which lies in the mid-dorsal line immediately under the external dorsal covering. Indeed, this ventral position of the heart in the one group of animals and its dorsal position in the other, combined with the corresponding positions of the central nervous system, is one of the principal reasons why all the advocates of the origin of vertebrates from the Appendiculata, with the single exception of myself, feel compelled to reverse the dorsal and ventral surfaces in deriving the vertebrate from the invertebrate. But there is one most important fact which ought to make us hesitate before accepting the homology of the dorsal heart of the arthropod with the ventral heart of the vertebrate—The heart in all invertebrates is a systemic heart, i.e. drives the arterial blood to the different organs of the body, and then the veins carry it back to the respiratory organ, from whence it passes to the heart.

Fig. 69.—Comparison of Branchial Lamellæ of Limulus and Scorpio with Branchial Lamellæ of Ammocœtes.

A, Branchial lamellæ of Scorpio (after Macleod); B, Branchial lamellæ of Ammocœtes (after Nestler).

The only exception to this scheme is found in the vertebrate where the heart is essentially a branchial heart, the blood being driven from the heart to the ventral aorta, from which by the branchial arteries it is carried to the gills, and then, after aeration, is collected into the dorsal aorta, whence it is distributed over the body. The distributing systemic vessel is the dorsal aorta, not the heart which belongs essentially to the ventral venous system. This constitutes a very strong reason for believing that the systemic heart of the invertebrate is not homologous with the heart of the vertebrate. How, then, did the vertebrate heart arise?

Let us first see how the blood is supplied to the gills in Limulus.