124 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



phragm undergoes many shiftings of position before reaching its final 

 place. 



The tunica serosa lining the various divisions of the splanchnocoele has special 

 names in each. Thus the pericardial and pleural cavities are lined by peri- 

 cardium and pleura respectively, that portion of the pericardium covering the 

 heart being sometimes called the epicardium. The metaccele or peritoneal cavity 

 is lined by the peritoneum. 



The metaccele is not always cut off completely from the external 

 world. In the lower vertebrates the urinary ducts frequently open into 

 the body cavity by the nephrostomes (fig. 133), and in these and even 

 in the mammals the oviducts of the female connect the cavity with the 



FIG. 133. Diagram of possible connection of coelom with the exterior, modified from 

 Bles. c, ccelom; d, cloaca; g, glomerulus of kidney; *, intestine; n, nephrostome; pa, 

 porus abdominalis. 



exterior. In many fishes there are pori abdominales leading from the 

 metaccele to the outside near the vent. These may be single or paired 

 and are found in cyclostomes, many elasmobranchs and teleosts, 

 ganoids, and dipnoi. None are known in amphibia, birds or mammals, 

 but in turtles and crocodilians so-called peritoneal canals occur, 

 usually ending blindly in chelonians, but emptying into the cloaca 

 in the crocodiles. These may be homologous with the abdominal 

 pores, but only the development can settle the question. In some 

 fishes the pores serve for the escape of the genital products; in other 

 animals their function is uncertain. 



THE MUSCULAR SYSTEM. 



Practically all motion in vertebrates is caused by muscles arising from 

 the mesoderm. While other cells may have a certain power of chang- 

 ing shape, the muscle cells possess this in a marked degree, and so 

 that they may cause the greatest amount of motion in the parts to which 

 they are attached, they are very long, stimulation causing them to 

 contract in length and at the same time to increase in diameter. 



