THE GIANT EARTH-WORM OF GIPPSLAND. 43 
studied, it will be impossible to establish any certain homologies. ‘Meanwhile, it is 
important to remember that in Chwtopods there is a very clear distinction of the 
nephridial duct into two parts—(1) an intracellular part, and (2) an intercellular part 
leading to the exterior, connected with which is the vesicular part. It may be that 
the first part is always mesoblastic in origin, the second epiblastic. In this case, the 
longitudinal duct and network of Megascolides and the network of other forms 
are mesoblastic, and hence different in origin from the duct present in the embryo 
Lumbricus. These points can only be determined by further, and especially 
embryological, investigations. The various stages in the development of the Chetopod 
nephridia may perhaps be somewhat as follows :— 
(1) A stage (in some Platyhelminth-like ancestor) in which in an unsegmented 
body a continuous network of nephridial tubules, with flame or internally-ciliated cells, 
the former uniting to form longitudinal canals leading to the exterior. 
(2) A modification (as seen in Dinophilus gyrociliatus) in which the excretory 
organs are still in the form of a network, with flame cells, but with secondary 
external openings in each segment, irregularly arranged, as in some planarians, 
regularly arranged, as in Dinophilus. 
(3) A further modification, resulting in the formation of numerous irregularly- 
arranged outgrowths from the nephridial network, having the nature of coiled tubules, 
which are directly continuous and identical in structure (duct intracellular) with the 
network. These form the nephridia of the more highly-developed worms. Their 
development is to be regarded as intimately associated with that of segmentally- 
arranged coelomic chambers, such as are at any rate but feebly represented in the 
Hirudinea, in which the tubes of the network itself, still lying within the mesoderm, 
become aggregated to form the nephridia. 
(4) In connection with these numerous nephridial tubes, many external openings 
leading into the still persisting network are formed. (P. aspergillwm.) 
(5) The aggregation of the small nephridia into groups, commencing in’ the 
posterior region of the body (as in A. multiporus and M. australis). The nephridia 
are sometimes aggregated in relationship to the sete, and as the aggregation 
proceeds so the external openings diminish in number and the network lessens in 
extent. 
(6) The formation of large nephridia either out of an aggregate of small 
nephridia, or by the special growth of one of an aggregation of small nephridia. Each 
large nephridium acquires secondarily an internal opening into the ccelom. These 
openings, which have a very definite relationship to the ccelomic chambers, are formed 
apparently later in the developmental history of the nephridia in Chwtopod than 
Hirudinea, and cannot be supposed to be related to the flame cells, but to be new 
formations within the group. Their formation also proceeds from the posterior 
towards the anterior end, and as it goes on the small nephridia gradually diminish in 
Ga 2 
