. Elements of Comparative Anatomy. iro COMPAEATIVE ANATOMY. vascular ramifications. A breaking-up such, as affects tlie parietal transverse anastomoses in tlie gills^ may affect also the longitudinal trunksj which then take the form of a continuous network of vessels, from which new passages are developed. The ph^eno- mena, which give rise to a collateral circulatiouj must also furnish us with the explanation of these relations. Thus in Poly- ophthalmus the median trunk is broken up along the course of the mid-gut. Two dorsal and two ven- tral trunks arise from the anterior and posterior simpl
. Elements of Comparative Anatomy. iro COMPAEATIVE ANATOMY. vascular ramifications. A breaking-up such, as affects tlie parietal transverse anastomoses in tlie gills^ may affect also the longitudinal trunksj which then take the form of a continuous network of vessels, from which new passages are developed. The ph^eno- mena, which give rise to a collateral circulatiouj must also furnish us with the explanation of these relations. Thus in Poly- ophthalmus the median trunk is broken up along the course of the mid-gut. Two dorsal and two ven- tral trunks arise from the anterior and posterior simple median vessels in the HermellidEe; in Eunice the ventral, and in Nephthys the dorsal, pair are present. In some the vascular system is atrophied (Grly- cera, Capitella). A combination of the types of the vascular sys- tem, which obtain in the Annelides and the Ne- mertines, can be made out in Balanoglossus. This consists in the pre- sence of both median and lateral longitudinal trunks, but their visceral branches partly function as branchial vessels, and so give rise to an arrangement which is very different from that of the majority of Vermes. § 140. The vascular system of the G-ephyrea presents characters which, not only in their relations to the circulatory system of other Vermes, but even when we compare their various conditions with one another, are by no means easy to understand; at the same time there are many important lacunee in our knowledge of the ana- tomical facts. The most important of these is the question whether there is any connection between the cavities of the vascular system. Fig. 79. Vascular system of Terebella nebu- losa; opened from the dorsal surface, t Tentacles (not fully drawn), hr Three pairs of branchise. p?i Muscular portion of the fore-gut (pharynx). V Enteron. vd Dorsal vessel, vv Yentral vessel (after Milne-Edwards).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability -
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Keywords: ., bo, bookcentury1800, bookdecade1870, bookpublisherlondonmacmillan
