40 ON THE ANATOMY OF MEGASCOLIDES AUSTRALIS, 
Both of these structures are present in what are to be, without much doubt, 
regarded as the more primitive form of nephridia in Chwtopods; whilst in more 
modified ones, the second set of structures is present only. 
Of the two, the first is to be regarded as directly homologous with the network 
present in Platyhelminthes, whilst the second is a secondary development. 
This gives a somewhat new view as to the relationship existing between the 
various nephridia of the groups, and differs from that of Brpparp and others in 
regarding the highly developed nephridia of such forms as, for example, Lwmbricus, 
as secondary developments, formed primitively as outgrowths from a nephridial 
network homologous with that of Platyhelnunthes and still persisting in Pericheta, 
their existence being intimately associated with the formation of definite sinuses 
and- spaces within the mesoderm. A series of gradations is found amongst 
adult forms in the number, structure, and arrangement of the nephridia. In what 
may be regarded as the more primitive ones—A. multiporus, P. aspergillum, &&.— 
there is present a regular network of ducts with coiled nephridial tubules depending 
from it into the ccelom, and very many in number in each seoment. In P. aspergillwm 
the simplest form is seen in which the nephridia are quite irregularly scattered, and 
the network is a continuous one from segment to segment. In A. multiporus an 
ageregation of the nephridia takes place in the posterior region of the body, with a 
consequent lessening of the extent of the network, which is not continuous from 
seoment to segment. In the more highly developed forms the nephridia 
are restricted in number, usually, though not always, (as, for example, in Capztellide 
amongst the Polychetw) asingle pair being present in each segment, as in Lumbricus. 
Each nephridium is in Chwtopods considerably larger and more highly developed 
than in the case of those where many are present. In one adult Polychwte—Lanice 
conchilega*—a longitudinal duct connecting consecutive nephridia exists in the adult. 
A similar (?) longitudinal duct has been described as present in the embryo of 
Lumbricus and Criodrilus, placing the various nephridia in connection with one 
another, and Harscunk’s well known description of the development of the nephridia 
in Polygordius indicates a definite connection between the successive ones in this 
form. : 
Between the two extreme forms, in one of which a complete network with very 
numerous small nephridia with no internal openings are present, and the other, in 
in which a pair of large nephridia with internal openings is developed in each 
segment in no serial connection with each other, at all events in the adult, a most 
important intermediate stage is found in Megascolides australis. ‘This serves in a 
remarkable manner to bridge over the wide interval, and to show the path by which 
the nephridia of Luwmbricus may perhaps have been developed from the primitive 
original network of ducts in a Platyhelminth ancestor. It possesses both large and 
* CunnincHAamM. Q,.J.M.S. Nov. 1887, p. 250. 
