! 3 



574 COMPARATIVE ANATOMY. 



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Ccelom. 

 § 428. 



In all Vertebrata, just as in many of the Invertebrata, a hollow 

 space is differentiated around the enteron of the trunk ; this is 

 effected by the cleavage of the mesoderm. It is, therefore, a 

 space which appears in the middle germinal layer, and separates the 

 epithelial lining of the digestive tract and the parts developed from 

 it from the organs differentiated from the outer germ-layer. The 

 limitation of this process of separation to the region of the trunk 

 appears to be correlated with the formation of branchial clefts in 

 the cephalic enteron, for this prevents any continuation forwards of 

 the cleft, at any rate laterally. As in the Invertebrata, the ccelom 

 forms a cavity, which is a part of the vascular system, in so far, that 

 is, as the lymphatic portion is connected with it. The direct com- 

 munication with the exterior which obtains in many Invertebrata 

 is not completely wanting, although it is not greatly developed. It 

 is represented by an abdominal pore which is placed near the 

 anus, and which is generally paired ; this is seen in the Cyclostoma 

 and among Gnathostoma also, in the Selachii, Chimasras, Oeratodus, 

 and many Teleostei ; it is last seen in the peritoneal canals of the 

 Crocodile. In the Chelonii there are merely indications of it. We 

 should also note that there is a free communication between the 

 ccelom and the excretory apparatus, for a connection with the con- 

 dition of lower organisms is thereby demonstrated (vide Excretory 

 organs) . 



The whole of the inner surface of the ccelom is invested by a 

 layer of epithelium, which is greatly developed in a certain region, 

 where it forms the germinal epithelium. The female germinal 

 glands are differentiated from this tract. In the anterior portion of 

 the ccelom of the lower divisions ciliated epithelium is distributed 

 over certain regions. The epithelium of the ccelom unites with a 

 subjacent layer of connective tissue to form a special membrane, the 

 peritoneum, which is continued on from the walls (where it forms 

 the parietal layer) to the parts (viscera) which are placed in, or 

 project into the ccelom; and these parts it also covers (visceral 

 layer) . 



In the Anamnia, the ccelom is a single cavity, as it is also in 

 most of the Reptilia, although in the Crocodilini we may observe 

 that an anterior portion is being separated from a hinder one. In 

 the Mammalia this separation is complete. The diaphragm separates 

 the hinder portion of the ccelom, or abdominal cavity, from an 

 anterior one, which contains the two lungs, and which is divided into 

 two lateral halves (pleural cavities), which contain the lungs, by a 

 median wall or partition, which contains the pericardium. 



