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 specialisation of the nephridia 
appears to commence posteriorly. The small and large ne- 
phridia are connected by a longitudinal duct. 
The next stage towards the condition found in Lumbricus 
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 Lumbricus (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 
