STRUCTURE OF ORGANISMS 



441 



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- 



l^Posten'or 



A Tubules D E 



Fig. 294. — 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, 

 with simple alveoli. C, lung of a lizard, showing increasing oomplexity. 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, Ajner. 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 closed 

 type, where the blood is confined within a closed system of vessels; and 

 the other the open type, where the blood circulates through sinuses 

 which are hemocoelic. Nemertines and annelids have a closed system; 

 in mollusks and arthropods the system is open; and in echinoderms and 

 chordates it is again closed. 



477. Excretory System. — In protozoans the only eliminative struc- 

 ture is the contractile vacuole. Sponges, coelenterates, and ctenophores 



