CHORISTOPODA. (395 



the thorax, while in the Isopoda they are abdominal, might seem to 

 settle the question in favour of the former, as thoracic branchise cha- 

 racterize all the higher Crustacea. But, as we have observed in 

 other places, general structure in the body takes precedence of cha- 

 racters of the above kind. The position of the branchial appendages 

 may confirm the conclusions from structure, but cannot set them aside. 

 The main facts upon which we rest, are those pertaining to the appen- 

 dages, and these appear to show that the Isopoda and Amphipoda have 

 a relation to one another, analogous to that of the Brachyura and 

 Macroura. 



a. In the first place, the Isopoda have the abdomen quite short, 

 with no proper feet, and instead, only branchial leaflets, besides a 

 single pair of stylets, which last are sometimes wanting. 



b. Again, the antennae are commonly very short, as in the Bra- 

 chyura. 



c. Again, the anterior set of legs includes three pairs instead of four, 

 the addition of the fourth pair to the anterior three, where it occurs, 

 being an evidence of a less concentration of force in the cephalic gan- 

 glions, precisely as the addition of the third pair to the series of chelate 

 legs, places the Penaei below the Palaemons. 



In each of these points the Amphipods show inferiority. 



1. They have an elongated abdomen, with natatory appendages 

 below, and are thus analogous to the Macroura. 



2. The antennae are usually quite long, also a Macroural charac- 

 teristic. 



3. The anterior set of legs includes four pairs. 



In addition, we observe, that the body in the Isopoda has in every 

 part a more compact aspect. Moreover, the species are often terres- 

 trial, a fact, as Prof. Agassiz has shown, marking the higher grades 

 among classes or groups of species. 



A significant fact of still higher moment is presented by the ner- 

 vous system. The Amphipods have often the two dorsal cords 

 distinct between the ganglia, while in the Isopods, there is but a 

 single cord. The Amphipods have thus their less concentration of 

 nervous influence and general force exhibited prominently in the ner- 

 vous system itself. This double cord is seen in none of the higher 

 Crustacea, and as we descend in the scale, first makes its appearance 

 in the Amphipoda. 



The Caprellidae among the Amphipods, appear to militate with the 



