. Animal biology. Zoology; Biology. STRUCTURE OF ORGANISMS 469 cells; and in the other phyla, by canals or branches of the gastrovascular cavity. The circulatory system when it does appear is formed in the mesenchyme, both the cells of the walls of the vessels and the blood corpuscles being modified mesenchymal cells appropriately arranged. The vessels become lined with endothelium, a type of epithelium also derived from the mesenchyme, and to this is added connective tissue and nonstriated muscles, making up the wall of the vessel. No true circulatory system appears in the nemathelminths or r
. Animal biology. Zoology; Biology. STRUCTURE OF ORGANISMS 469 cells; and in the other phyla, by canals or branches of the gastrovascular cavity. The circulatory system when it does appear is formed in the mesenchyme, both the cells of the walls of the vessels and the blood corpuscles being modified mesenchymal cells appropriately arranged. The vessels become lined with endothelium, a type of epithelium also derived from the mesenchyme, and to this is added connective tissue and nonstriated muscles, making up the wall of the vessel. No true circulatory system appears in the nemathelminths or rotifers, but a blood-vascular system is found in nemertines, bryozoans, brachio- pods, echinoderms, and higher forms. In the primitive stages of the blood-vascular system as it appears in the lower animals pulsations occur throughout the system. In the higher forms, however, the pulsa-. A Fig. 352.—Diagrams illustrating the increase in the amount of surface exposed to the air in different types of lungs. A, lung of Necturus, without alveoli. B, lung of a frog, wath simple alveoli. C, lung of a lizard, showdng increasing complexity. D, lung of a bird, seen from the inner side, showing the bronchus entering anteriorly and the passage to the air sacs posteriorly; the passages in the lungs are seen to form a continuous system of tubules, none of which ends blindly. E, lung of a mammal showing the branching of the bronchi. F, a portion of the bronchi of a mammal, to show the ending in alveoli. {D from Locy and Larsell, Amer. Jour. Anatomy, vol. 20.) tions become limited to certain structures called hearts, which are dilated chambers provided with a larger amount of muscular tissue than exists in vessels generally. A single heart is found in all arthropods, in mol- lusks, and in chordates with the exception of Hemichordata. In verte- brates there has been seen a gradual increase in the number of chambers in the heart. There are two types of circulatory systems, one known as the clo
Size: 2342px × 1067px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcoll, booksubjectbiology, booksubjectzoology
