76 ZOOLOGY. 



which generally runs straight through the body, without 

 convolutions, until it reaches the anal opening. 



101. Circulation. " No amielide ever possesses a heart 

 comparable to the heart of a crustacean or insect ; but a 

 system of vessels, with more or less extensively contractile 

 walls, containing a clear fluid, red or green in colour, 

 and, in some cases only, corpusculated, is very generally 

 developed, and sends prolongations into the respiratory 

 organs, when such exist" (Huxley). 



Fig. 32. SECTION OF AN ANNELIDE. 



This circulation is termed pseudo-hasmal (Gr. pseudos, 

 false ; haima, blood) ; and the vessels that contain this red 

 or green fluid are considered extreme modifications of the 

 "water vessels" of the Annuloida. There is also a cor- 

 pusculated fluid contained in the spaces surrounding the 

 internal organs (the peri visceral cavity), which is be- 

 lieved to correspond with the true blood system of the 

 Arthropoda. 



102. Respiration. The breathing organs are either 

 external gills of various forms, or bladder-like sacs placed 

 on the sides of the body. " The external gills are 

 well seen in the common lug- worm, where they consist of 

 eleven pairs of arborescent organs, placed externally, and 

 rather in advance of the middle line of the body. ' ' (Haughton. ) 



In some cases there are no special breathing organs, 

 respiration being effected by the surface of the body. 



103. The nervous system is of the usual annulose type. 

 It consists of a double chain of ganglia running along 

 the ventral part of the body, the gullet passing between 



