ANNELIDA 83 



and thus forms an efficient means of gaseous exchange. The 

 skin is the respiratory organ. The parietal vessels pour their 

 blood into the dorsal vessel (figs. 87 and 88). 



Excretory System. It has already been noted that the 

 yellow qells of the wall of the coelom are concerned in excre- 

 tion, but.the kidneys reach a high degree of development. A 

 pair of nephridia is present in each segment except the first 

 three and the last. The nephridia are derived from ectoderm, 

 and each is a tube which ends in an opening into the coelom. 

 The opening internally is the nephrostome. It is funnel- 

 shaped, ,and the rim is provided with long cilia which are 



Nephrostome Glandular seg. Thin-walled seg. 



Nephridiopore Muscular seg. 



FIG. 39. Lumbricus. Diagram of nephridium. 



incessantly moving. It projects from the anterior face of a 

 septum in each case, and as it pierces the septum it narrows 

 to form a tube the cells^of which are bent round to enclose the 

 cavity and are ciliated. This tube is disposed in folds close to 

 the septum, but it is on the opposite side to that containing 

 the nephrostome. It expands into a glandular segment which 

 is likewise folded into loops. The glandular segment is con- 

 tinued into a muscular segment which leads to the nephrido- 

 pore. The loops formed are complicated, but will be easily 

 followed in fig. 39. The blood-vessels are distributed over 

 the tubes in fine branches and more especially to the glandular 

 segment. It is in this region that the nitrogenous waste 

 material is removed from the blood by the cells, and the 

 coelom provides a head of watery fluid which washes the 

 material to the lower part of the tube. With this in mind, 

 it is clear that the excretion is still to a large extent in the 



