128 THE APODID^ part i 



an opening. Its redevelopment in the higher Crus- 

 tacea is paralleled by the redevelopment of the 

 dorsal parapodia of the mandibles, although the 

 latter had disappeared in Apus. It is a well-known 

 principle that organs which have disappeared may 

 reappear in the descendants of those who have learnt 

 to dispense with them. 



The absence of the antennal gland in Apus is 

 perhaps to be explained by the enormous size of the 

 shell gland. In one specimen of L. Spitzbergensis, 

 1 1 mm. long to the tip of the caudal plate, the coils of 

 the shell gland on each side measured over 25 mm. 

 Such an enormous gland would no doubt be able to 

 undertake the greater part of the excretion of the 

 body. 



We have thus, in the Crustacea, three setiparous 

 glands preserved in the head : the salivary (?) gland 

 of the first antennae (in Apus at least), the antennal 

 gland, and the shell gland of the second maxillae. 



The antennal glands as well as the shell glands are 

 generally homologised with the Annelidan nephridia. 

 From the foregoing account of the origin of these 

 glands we repeat the following points, which must 

 render such a homology improbable. 



(i) The position of the external opening is on 

 the dorsal parapodium, — an unlikely place for the 

 opening of a nephridium, but, on the other hand, 

 quite a proper place for the opening of an acicular 

 gland. 



(2) The structure of the glands as we find them (in 

 Apus at least) is exactly that of setiparous glands. 



