MORPHOLOGY OF ANIMALS 

 6. Circulatory System 



The physiological significance of the circulation of fluids 

 within the body is the distribution of nutriment and, in 

 some cases, of oxygen to all parts. In the simplest Metazoa 

 (Cnidaria, Ctenophora) there is no need of a special cir- 

 culatory apparatus other than that which is furnished by 

 the gastric cavity itself; this may branch and extend to 

 various parts of the body of a jellyfish or hydroid colony, 

 thus forming a g astro-vascular system^ through which the 

 distribution of nutriment takes place; the branched gastric 

 cavity of certain turbellarians also serves a similar func- 

 tion (Fig. 11, A). Circulation of body fluids also occurs 

 in many lower animals without the aid of any special cir- 

 culatory apparatus; in such cases lymph, containing the 

 products of digestion, is distributed through all the inter- 

 cellular spaces in the primary body-cavity, and by the 

 contractions of the general musculature of the body it is 

 kept in irregular movement. 



Blood-Vascular System. With the single exception of 

 the nemerteans a blood-vascular system is found only 

 among animals with a secondary body-cavity, or true 

 coelom, and is lacking even in some of these, particularly 

 such as are quite small or are evidently degenerate forms. 

 With a few exceptions it is present in mollusks, echino- 

 derms, annelids, arthropods, and all chordates. In its sim- 

 plest form it consists of branching and anastomosing tubes 

 which contain blood. The walls of the tubes are composed 

 of flattened epithelial cells (endothelium) which may be 

 surrounded on the outside by muscle and connective tissue 

 fibres. The blood which fills these vessels consists of a 



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