Introduction to Animal Morphology. 31 



The alimentary canal may be the only body cavity, 

 or it may be suspended in a larger separate cavity 

 (Coeloma) formed either by outgrowing spaces from 

 itself (Ctenophora, Echinodermata, &c.), or by in- 

 vagination (Amphioxus), or by the splitting and 

 separation of two mesoblastic layers. 



In simple animals the nourishment is taken into 

 the plastides of the stomach wall, but as these walls 

 become membranous, and as other organs become 

 more complex, the assimilated fluid collects in separate 

 spaces where, by osmose or the general contraction of 

 the body wall, or by a special contraction of a 

 muscular tube, it is made to move so as to bathe the 

 tissues. This newly-formed system is the nutritive or 

 circulatory. At first, as in Actinozoa, with a body 

 cavity (distinct from the stomach), it is in this space 

 that the nutritive fluid, mixed with water, is to be 

 found. In the lower Worms, where the digestive and 

 perivisceral spaces do not communicate directly, the 

 body cavity still contains the nourishing fluid, which 

 yet often contains sea water. In the higher Annulosa 

 definite vessels exist, but yet in some the blood only 

 traverses these for a limited part of its circulation, as 

 at each end they often freely communicate with the 

 perivisceral spaces. In some Arthropods and Molluscs 



and taurocholatc, bilirubin (< . biliverdin (CieHN .;<>..). as 



;lic bile contains hyocholic and hyocholeic acids, in 



.11, taurinc ' M.nholic acid is C 2 ,-.I I . \< >... t.iurocholic, 



CjHi.-,NSi ) : . J'.ilc j'tecijiit.ites peptone^. In the k'all bladder the bile is 



thickened bjr the addition of nun in. The bile i-, largely excrenentitioas, 



but . .:\in^ f.its. The pigments .ire formed of 



alter- i tluid containing water. 



] hymatin. -in. lemin, ^uaniti in ti.u 68, It c inuKilies 



tl their absorption, dissoh cs albunii- 







