The Origin of Vertebrates

by Walter Holbrook Gaskell

Fig. 70.—Longitudinal Diagrammatic Section through the Mesosomatic Region of Limulus, to show the origin of the Branchial Arteries. (After Benham.)

L.V.S., longitudinal venous sinus, or collecting sinus; a. br., branchial arteries; V.p., veno-pericardial muscles; P., pericardium.

In Limulus the blood flows into the lamellæ from sinuses or blood-spaces (b.s., Fig. [66]) at the base of each of the lamellæ, which sinuses are filled by a vessel which may be called the branchial artery, since it is the afferent branchial vessel. On each side of the middle line of the ventral surface of the body a large longitudinal venous sinus exists, called by Milne-Edwards the venous collecting sinus, L.V.S., (Fig. [70] and Fig. [58]), which gives off to each of the branchial appendages on that side a well-defined afferent branchial vessel—the branchial artery (a. br.). The blood of the branchial artery flows into the blood-spaces between the anterior and posterior laminæ of the appendage and thence into the gill-lamellæ, from which it is collected into an efferent vessel or branchial vein, termed by Milne-Edwards the branchio-cardiac canal, which carries it back to the dorsal heart. The position of the branchial artery and vein is shown in Fig. [66], which represents a section through the branchial appendage of Limulus at right angles to the cartilaginous branchial bar (br. cart.), just as Fig. [65] represents a section through the branchial appendage of Ammocœtes at right angles to the cartilaginous branchial bar.

Further, the observations of Blanchard, Milne-Edwards, Ray Lankester, and Benham concur in showing that in both Limulus and the scorpion group a striking and most useful connection exists between the heart and these two collecting venous sinuses, in the shape of a segmentally arranged series of muscular bands (V.p., Fig. 70 and Fig. [58]), attached, on the one hand, to the pericardium, and on the other to the venous collecting sinus on each side. These muscular bands, to which Lankester and Benham have given the name of 'veno-pericardial muscles,' are so different in appearance from the rest of the muscular substance, that Milne-Edwards did not recognize them as muscular, but called them 'brides transparentes.' Blanchard speaks of them in the scorpion as 'ligaments contractiles,' and considers that they play an important part in assisting the pulmonary circulation; for, he says, "en mettant a nu une portion du cœur, on remarque que ces battements se font sentir sur les ligaments contractiles, et determinent sur les poches pulmonaires une pression qui fait aussitot refluer et remonter le sang dans les vaisseaux pneumocardiaques." Lankester, in discussing the veno-pericardial muscles of Limulus and of the scorpions, says that these muscles probably contract simultaneously with the heart and are of great importance in assisting the flow through the pulmonary system. More recently Carlson has investigated the action of these muscles in the living Limulus and found that they act simultaneously with the muscles of respiration.

Precisely the same arrangement of veno-pericardial muscles and of longitudinal venous collecting sinuses occurs in the scorpions. It is one of the fundamental characters of the group, and we may fairly assume that a similar arrangement existed in the extinct forms from which I imagine the vertebrate to have arisen. The further consideration of this group of muscles will be given in Chapter IX.

Passing now to the condition of the branchial blood-vessels of Ammocœtes, we see that the blood passes into the gill-lamellæ from a blood-space in the appendage, which can hardly be dignified by the name of a blood-vessel. This blood-space is supplied by the branchial artery which arises segmentally from the ventral aorta (V.A.), as seen in Fig. [71] (taken from Miss Alcock's paper). From the gill-lamellæ the blood is collected into an efferent or branchial vein (v. br.), which runs, as seen in Fig. [65], along the free edge of the diaphragm, and terminates in the dorsal aorta.

The ventral aorta is a single vessel near the heart, but at the commencement of the thyroid it divides into two, and so forms two ventral longitudinal vessels, from which the branchial arteries arise segmentally.

Fig. 71.—Diagram constructed from a series of Transverse Sections through a Branchial Segment, showing the arrangement and relative positions of the Cartilage, Muscles, Nerves, and Blood-Vessels.

Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured blue are the internal sensory nerves to the diaphragms and the external sensory nerves to the sense-organs of the lateral line system. Br. cart., branchial cartilage; M. con. str., striated constrictor muscles; M. con. tub., tubular constrictor muscles; M. add., adductor muscle; D.A., dorsal aorta; V.A., ventral aorta; S., sense-organs on diaphragm; n. Lat., lateral line nerve; X., epibranchial ganglia of vagus; R. br. prof. VII., ramus branchialis profundus of facial; J.v., jugular vein; Ep. pit., epithelial pit.