CHORISTOPODA. 693 



In these particulars we observe two distinct types of structure of 

 fundamental character; and any species which do not partake of these 

 peculiarities, or such as are intermediate forms, partly having the 

 characteristics of the Isopoda and partly those of the Amphipoda, 

 may be arranged in a distinct group. 



Of these intermediate forms, the group Anisopoda consists. They 

 have 



1. Like Amphipoda, the three posterior pairs of thoracic legs in one 

 series, and the four anterior in a different series. 



2. Like Isopoda, the three posterior pairs of abdominal members 

 are not styliform, only the last having this character. 



These are constant characters, distinguishing the whole group. But 

 there are various degrees of divergence from either the Amphipoda or 

 Isopoda, or of convergence towards one or the other tribe, which 

 mark out their affiliations, and show the true transition character of 

 this section of Choristopods. 



In some species (Arcturi) all the abdominal appendages are pre- 

 cisely as in the Isopods, although Amphipodan in the thoracic appen- 

 dages, and at the same time the abdomen has the joints hardly 

 flexible, instead of allowing free motion, as in the Amphipods. 



In others, not far remote (species of Tanais, male Bopyri, etc.), the 

 abdomen is more elongated; the joints have some free motion; the 

 appendages are but imperfectly branchial, the branches being elon- 

 gated and long ciliated, as in the Amphipods, and moving freely be- 

 neath the somewhat elongated abdomen. Yet although so much like 

 Amphipods, as, in the case of one or two genera, to have been referred 

 to the Amphipoda rather than the Isopoda, the fourth and fifth pairs 

 of abdominal appendages are not styliform like the sixth, as in the 

 Amphipods; the sixth, on the contrary, is Isopodan in character, and 

 the fourth and fifth, in Isopod style, hardly differ from the second and 

 third. Thus, while these appendages are nearly Amphipodan in struc- 

 ture (excepting the sixth), instead of there being only three pairs sub- 

 natatory, all five pairs have a similar character, so that the members 

 in the two series have the Isopod number, 5 : 1, and not 3:3. 



In other species, the abdomen diverges from the Isopod form in 

 another manner. It gives the Amphipod ratio, 3:3; and the three 

 anterior pairs have nearly the Amphipod form, being elongated and 



174 



