148 BIOLOGY: GENERAL AND MEDICAL 



appears Jis a distinct function. It is not, however, 

 quite so simple a function as would appear at first 

 sight, for upon examination it is found that the fine 

 branches of the water vascular system by which the cir- 

 culating pabulum is conveyed to the surface do not 

 terminate in simple openings, but in specialized cells 

 of the dermal covering of the animal, known as " flame- 

 cells" so-called because an appearance suggesting 

 the flickering of a flame is caused by the movement of 

 vibratile cilia situated in vacuoles of these cells. The 

 tubules terminate in vacuoles of these cells which seem 



FIG. 55. Diagram of a nephridium (simple kidney tubule) of a segmented 

 worm. b,b l , blood vessels; c, coelome; d, duct of the nephridium; e, external 

 opening; cf, ciliated funnel opening into coelome; gl, glandular or secreting 

 portion; a, septum; W, body wall composed of longitudinal muscle fibres, 

 circular fibres, and epithelial layer; w, wall of gut. (Gattoway.) 



to carry on the excretory function. It is not a mere 

 percolation of fluid through pores with which we have 

 to do, but a function of certain specialized cells. 



In the annulata we find special organs of excretion, 

 known as nephridia, a pair of which is found in each 

 segment. Each nephridium consists of a much con- 

 voluted epithelial-lined tubule which begins in a funnel- 

 shaped ciliated orifice opening into the ccelomic cavity 

 of the animal and directed anteriorly. This gathers up 

 the body fluids and passes them through the convoluted 

 portion of the tubule from which they eventually escape 



