504 



STRUCTURE OF VERTEBRATA. 



It is probably never quite closed, but may communicate with the 

 exterior by abdominal pores (or through nephrostomes) opening into 

 the renal system. Both occur together in some Elasmobranchs, but 

 they are usually mutually exclusive. In the higher Teleostei, in some 

 Saurians, and in Mammals, there are neither abdominal pores nor 

 nephrostomes, but only openings (stomata) into the lymphatic 

 system. 



Vascular system. From Cyclostomata onwards the 



blood fluid contains red cor- 

 puscles, i.e. cells coloured 

 with hsemoglQbin a pigment 

 which readily forms a loose 

 union with oxygen, and bears 

 it from the exterior (through 

 gills or lungs) to the tissues. 

 These pigmented cells are 

 usually oval and nucleated. 

 In all Mammals except 

 Camelidae they are circular. 

 Moreover, the full-grown red 

 corpuscles of Mammals have 

 no visible nuclei. The blood 

 fluid also contains uncoloured 

 nucleated amoeboid cells, the 

 white corpuscles or leuco- 

 cytes, of much physiological 

 importance. Some of them, 

 specialised as phagocytes, 

 form "a body-guard," at- 

 tacking and destroying micro- 

 organisms within the body. 



The heart receives blood 

 from veins, and drives it forth 

 through arteries. Its contrac- 

 tions in great part cause the 

 inequality of pressure which makes the blood flow. It lies 

 in a special part of the body cavity known as the peri- 

 cardium, and develops from a single (sub-pharyngeal) 

 vessel in Cyclostomata, Fishes, and Amphibians, from a 

 pair in Reptiles, Birds, and Mammals. 



The receiving region of the heart is formed by an auricle 

 or by two auricles ; thence the blood passes into the 



FIG. 268. Blood corpuscles. 



i. Amphibian, seen on the flat, oval, 

 bi-convex disc (nucleated) ; 2, am- 

 phibian, in profile ; 3, mammalian 

 (non-nucleated), circular, bi-concave 

 disc ; 4, mammalian, in profile ; 5, 

 camel's (non - nucleated), oval ; 6, 

 mud -fish (Lepidosiren) in section, 

 like Amphibian ; 7, Lepidosiren, seen 

 on the flat ; 8, an amoeboid leucocyte 

 with lobed nucleus and large gran- 

 ules ; 9, a leucocyte with non-lobed 

 nucleus and minute granules ; 10, a 

 leucocyte dividing into two; n, a 

 flat amoeboid corpuscle or blood 

 platelet 



