COXAL GLAND OF LIMULUS AND OF ARACHNIDA. 517 
The view that the genital ducts of the Arthropoda are modi- 
fied nephridia is further supported by the consideration that 
there is no other plausible suggestion as to their origin and 
significance. From this point of view we have to bring into 
consideration all animals whose genital ducts are continuous 
with the gonads and open to the exterior. Animals are, as I 
have elsewhere pointed out (‘Encycl. Brit./ article “Mollusca”), 
either Schizodinic or Porodinic, that is, discharge their genital 
products by rupture or by permanent pores. The Porodinic 
forms are, according to our present knowledge, divisible into 
those which are nephrodinic and those which are idiodinic, the 
ducts being in the first case “ nephro-gonaducts,” and in the 
second case “ idio-gonaducts.” But it seems possible that such 
a thing as “ idio-gonaducts” have no real existence. The 
gonad itself in Ccelomate animals is essentially a group of cells 
forming part of the lining of the coelom or body cavity, and it 
seems quite likely that in all cases the duct, even when it is 
intimately fused with the gonad, was primitively a nephridium. 
If this is universally true we have to reckon nephridia as form- 
ing gonaducts by fusion with the gonads in Echinoderms, in 
Platyhelminthes, and in Nematoid worms, as well as in Arthro- 
poda and Mollusca — cases which at present are regarded as 
typical instances of the occurrence of idio-gonaducts. 
Possibly the generalisation may not prove to be justified in 
all these groups, whilst holding for the Arthropoda and 
Mollusca. 
The full consideration of the suggestion here made involves 
a more definite conception than we at present possess of the 
nephridium as a primary organ of the ancestral Coelomate. 
How many pairs of nephridia may we assign to that ancestral 
animal? Is every tubular structure opening from coelom to 
exterior necessarily to be considered as belonging to one cate- 
gory — the nephridium? How can pores such as the dorsal 
pores of the Earthworm be distinguished from rudimentary 
nephridia ? If pores leading from the coelom to the exterior 
have an existence independently of nephridia, how are we to 
distinguish those pores which are merely “ reduced” nephridia 
VOL. XXV. NEW SER. 
L L 
