456 



STRUCTURE OF VERTEBRATA. 



Body cavity. — In Amphioxus a paired pouch grows out 

 from the archenteron. Almost at once this becomes 

 divided up on either side to form a series of small sacs, the 

 cavities of which form ultimately the true body cavity or 

 ccelom. According to Hertwig, this is in type the method 

 of formation of the ccelom throughout the Vertebrata. 

 In the other Vertebrates, owing to modified processes 



of development, probably 

 first arising from the pre- 

 sence of much yolk, solid 

 cell masses grow out in 

 place of hollow sacs, but 

 the cavities which appear 

 later, apparently by splitting 

 of the cell mass, are in 

 reality the retarded cavities 

 of true ccelom-pouches. 



The body cavity may form part 

 of one or all of the following 

 systems : — (i) excretory, void- 

 ing waste by abdominal pores or 

 by nephrostomes ; (2) reproduc- 

 tive, receiving the liberated 

 genital elements ; and (3) lym- 

 phatic, receiving transudations 

 from visceral and abdominal 

 organs. 



It is probably never quite 

 closed, but may communicate 

 with the exterior by abdominal 

 pores (or through nephrostomes) 



Fig. 194. — Transverse section through 

 a Teleostean embryo (diagram- 

 matic). — After Ziegler. 



s.c, Spinal cord; N., notochord ; ao., 

 aorta ; c.v., cardinal veins (united); tf.rf., 

 segmental duct; c, ccelom or pleuro- 



peritoneal cavity ; v.v., position of opening into the renal system, 

 median vitelline vein; y., yolk; En., 

 endoderm of gut ; mt. t myotome. The 

 dots represent mesenchyme cells ; the 

 little circles, blood corpuscles. 



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 corpuscles, i.e. cells coloured with 

 haemoglobin — 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 Camelidas they are 



