COELOMATA 79 



either through the nephridia or by special ducts or apertures, 

 (iu.) It communicates with the exterior through the nephridia, 

 or the organs which excrete waste nitrogenous matter. 



It is prohable that the body-cavities or coeloms which 

 originate in different manners may not be homologous through- 

 out the Coelomata; on the other hand they all possess the 

 characteristics enumerated above. 



There are other cavities which arise in the mesoblast to 

 which the above characteristics do not apply. These are the 

 vascular and lymphatic systems ; they contain blood and 

 lymph. In some Coelomata these systems are composed of 

 vessels with certain muscular differentiations to propel the 

 contained fluid ; in others, they form large spaces which simulate 

 the appearance of the body-cavity of other animals. Such 

 spaces are termed pseudocoels or haemocoels ; they occur in 

 Arthropods and Molluscs, and when they are present the true 

 coelom is very much reduced. It is not impossible that the 

 vascular system is but a part of the general coelom ; in many 

 cases the two systems are distinct, but in others — as the Leeches, 

 where they communicate through the botryoidal tissue, and 

 in Vertebrates through the thoracic duct — there is an indirect 

 connection, whilst in Nemertines the coelom and vascular 

 system appear to be one. 



The mesoderm, which is such an important feature in the 

 Coelomata, occupies the same position between the ectoderm 

 and endoderm as the mesogloea of the Coelenterata ; it is not, 

 however, always regarded as homologous with the mesogloea, 

 partly because the latter appears originally as a clear gelatinous 

 layer which is devoid of cells, and may remain so throughout 

 life ; but also because cells, when they do wander into the meso- 

 gloea, do not arrange themselves in definite tissues. 



