MORPHOLOGY OF ANIMALS 



flattened mesoderm cells, the peritoneum^ and is usually 

 divided into right and left halves by two longitudinal par- 

 titions, the dorsal and ventral mesenteries^ one of which 

 lies dorsal to the alimentary canal, the other ventral to it 

 (^, %\ Fig. 5, C) ; in some animals, one or both of these mes- 

 enteries may disappear (Fig. 5, D). In segmented animals 

 the coelom may be further divided into a series of cham- 

 bers by transverse partitions, the dissepiments or septa 

 Figs. 3, D; 12, A), which may disappear more or less com- 

 pletely in some cases. The excretory and sexual organs 

 are developed in large part from the walls of the coelom 

 and project into its cavity. The portion of the coelom 

 surrounding the heart is usually separated from the re- 

 mainder and is called the pericardial cavity; while 

 in the highest vertebrates (mammals) the anterior portion 

 of the coelom which contains the lungs is separated by the 

 diaphragm from the posterior part containing the abdom- 

 inal viscera. 

 3. Metamerism. A further complication of the gastrula is 

 introduced in segmented animals by the repetition of the 

 principal organs of the body in a series, one behind the 

 other Fig. 3, D) ; such repetition is known as metameric 

 segmentation, and each segment of the body is called a 

 somite (annelids, arthropods, vertebrates). In the simplest 

 cases each somite has its own section of the coelom and 

 its own sensory, nervous, muscular, alimentary, respira- 

 tory, excretory, and sexual organs, and each may bear a 

 pair of limbs or locomotor organs. Each somite in short, 

 contains all the important organs and may properly be 

 called a little body (i.e. somite). 



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