COELOMATA 79 
either through the nephridia or by special ducts or apertures. 
Qu.) It communicates with the exterior through the nephridia, 
or the organs which excrete waste nitrogenous matter. 
It is probable 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. 
