CHAETOPODA 147 



front and open into the preceding segment by a well-developed 

 funnel. In Megascolides there are a great number of minute 

 nephridial tubules, consisting of a short straight tube and a 

 longer coiled tube, scattered all over the inner surface of- the 

 skin. These small nephridia have an intracellular duct, and 

 are well supplied with blood-vessels. They open to the 

 exterior, but no internal opening has been found. In addition 

 to these smaller nephridia, the posterior half of the body has 

 in each segment a pair of large nephridia, with an internal 

 funnel-shaped opening. When these large nephridia are 

 traced forward through the region of the middle of the body, 

 it is seen that they first lose their internal funnel, and then 

 gradually decrease in size, and ultimately merge into the 

 smaller nephridia. Thus the speciaUsation of the nephridia 

 appears to commence posteriorly. The small and large ne- 

 phridia are connected by a longitudiaal duct. 



The next stage towards the condition found in Zumhricios 

 is when the network becomes discontinuous at the septa, 

 and does not spread from segment to segment. This stage 

 is almost reached by Deinodrilus, and quite by Acanthodrilus 

 and Dichogaster. Then, as is shown in the case of P. armata 

 and Megascolides, certain of the tubules of the network enlarge, 

 and form large nephridia, and the network gradually ceases to 

 be formed. Two pairs of such large nephridia exist in each 

 segment in Brachydrilus; one pair then disappears, and the 

 condition of Zumhricus (Fig. 92) is attained. 



The aquatic Oligochaets. have one pair of nephridia in 

 each somite ; the funnel is absent in Chaetogaster. 



In certain land worms the nephridia of the anterior seg- 

 ments become modified, and undergo a very remarkable 

 change of function. In Acanthodrilus dissimilis, in Dicho- 

 gaster, and in Digaster, all three possessing a nephridial 

 network, some of the tubules on each side of the pharynx 

 become connected with a duct which opens into the buccal 

 cavity. In Megascolides we have a similar change of function. 

 The walls of the pharynx are pierced by a number of tubules, 

 with an intracellular lumen, which opens into the cavity of 

 the alimentary canal, and whose secretions pass into the 

 pharynx. In every respect these tubules resemble the tubules 



