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 splanchnoccele 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 
Cc l 
} °LLZELLZZZZZ ZIZZO 
Fic. 133.—Diagram of possible connection of ccelom with the exterior, modified from 
Bles. c, coelom; ci, cloaca; g, glomerulus of kidney; 7, intestine; x, nephrostome; fa, 
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. 
