116 ISOPODA. 



Group ABERRANTIA. 



THIS group is formed of those Crustacea which, al- 

 though belonging in certain important conditions to the 

 order Isopoda, offer peculiar aberrant characters. 



In the Amphipoda the branchial organs are (as in 

 the Podophthalmic orders) attached to the appendages 

 of the body. In the aquatic Isopoda they are attached 

 to the fourth and fifth pairs of the appendages of the 

 tail. But in the several genera which we link together 

 to form this group, the branchial organs have not been 

 made out ; indeed, both Dr. Fritz Miiller and Prof. 

 Lilljeborg assert that no such organs exist. 



In one genus alone (and possibly only in certain 

 individuals of that genus) we have detected a saccular 

 appendage attached to the coxa of the third pair of 

 pereiopoda (or fifth pair of legs), that can, we think, 

 only be interpreted as a branchial organ similar to those 

 which exist in the Amphipoda attached to most of the 

 legs. Again, in Amphipoda, the heart lies beneath the 

 dorsal surface of the body, in the Isopoda in the tail. 

 In Tanais and Apseudes, according to Dr. Fritz Miiller, 

 the heart is situated as in the Amphipoda. And in the 

 embryonic condition the development is after the manner 

 of the Amphipoda rather than that of the Isopoda. 



"We restrict this group to the typical genera of Dana's 

 sub-order Anisopoda. It also corresponds with Dr. 

 Milne Edwards' ASSELOTES HETEROPODES, to which we 

 have added the family of ANTHURID^E. 



This group, in consequence of the variation in habits, 

 we have divided into two tribes, VAGANTIA and SUB- 



PARASITICA. 



