. Elements of Comparative Anatomy. VASCULAR SYSTEM OF VEEMES. 169. Fig. 78. Diagrammatic transverse section through the hinder half of the body of Arenicola, to show the arrangement of the vessels. D Dorsal; Y Ventral side, n Ventral medulla. i Enteric cavity, hr Branchiae, v Ventral vascular trunk. ah Branchial vessels, d Dorsal vascular trunk. ]\ Branch surrounding the enteric canal, v' Visceral ventral vessel. vascular system in various portions of tlie body^ and it is only in some wliicli live in tlie mud of fresh water, and in wliicli the hinder part of the body takes a special share in r
. Elements of Comparative Anatomy. VASCULAR SYSTEM OF VEEMES. 169. Fig. 78. Diagrammatic transverse section through the hinder half of the body of Arenicola, to show the arrangement of the vessels. D Dorsal; Y Ventral side, n Ventral medulla. i Enteric cavity, hr Branchiae, v Ventral vascular trunk. ah Branchial vessels, d Dorsal vascular trunk. ]\ Branch surrounding the enteric canal, v' Visceral ventral vessel. vascular system in various portions of tlie body^ and it is only in some wliicli live in tlie mud of fresh water, and in wliicli the hinder part of the body takes a special share in respiration, that the parietal vascular coils present any very great develo^^ment (Lumbriculus). Among the Chfetopoda also these simpler relations obtain, but the greater differentiation of the head, and of the fore-gut, is followed by some changes in the vascular system. When branchiaa are present the parietal vascular system is continued into them; in the simplest case a loop of the vessel passes into the appendage, which has the function of a gill. Here we have the com- mencement of the gradual separation of an arterial and a venous portion. This arrangement is re- peated when the branchiae are distributed over a large number of metameres, as in Eunice and Ai-enicola. The dorsal trunk gives off, in addition to the vessels for the enteron, others which pass to the laterally-placed branchite; from each of these a vessel passes to the ventral trunk (Fig. 78). The same characters obtain in the Hermellidte, where the branchiae have but a single central cavity, and where, therefore, there can be no anatomical separation of the efferent and afferent blood. In Arenicola these characters obtain in the hinder half only of the body. In the anterior half one branchial vessel passes to the chief ventral trunk, and the other to a visceral ventral vessel. When the respiratory appendages are limited to a smaller portion of the body, as is the case in the Tubicola;, there is a corresponding i
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