RESPIRATORY ORGANS IN DECAPODOUS CRUSTACEA. 459 
either still lower on the Euphausia stem, since epipodites (see 
fig. 9) are often found on all the thoracic feet as well as gills ; 
or, as seems more probable, from some earlier form of the 
Lophogastridae which had epipodites, represented more fully 
than they now are, on all the thoracic limbs. In reference to 
this matter, the reader is referred to the genealogical tree on 
p. 4, which is nearly the same as that given by Claus with 
reference to other structures as well as the branchiae. He 
does not, however, discuss the special relations of the separate 
Schizopod groups either to each other or to the Decapods. 
In the Decapods the thoracic feet have no longer a swim- 
ming function. Consequently one branch of the biramose 
limb (the exopodite) has either become vestigial or is alto- 
gether wanting. The three anterior thoracic limbs have now 
become maxillipedes, instead of only one as in the Schizo- 
pods, while the hinder thoracic appendages (ix — xm) have an 
ambulatory function. The swimming function is therefore 
left to the swimmerets or appendages of the abdomen, which in 
the Schizopods were in all cases very small although as a rule 
present. In most of the Decapods these swimmerets also 
attain no very great size, and in the long-tailed forms the 
telson with the two appendages of the penultimate segment is 
largely used in swimming, while the short-tailed forms 
scarcely swim at all. This change in function of the appen- 
dages does not, however, affect the gills, since these have 
already become fixed to the thoracic region in the Schizopod 
stage, which is gone through both phylogenetically and onto- 
genetically by the Decapod. One change, however, though 
apparently a slight one, does affect them, and this is the 
increase in length of the epimeral walls, as may be seen by 
comparing the two diagrammatic sections of Gnathophausia 
(fig. 10) and Astacus (fig. 11). 
In consequence of the raising of the pleura the epimeral 
walls, and with them, it appears probable, the arthrodial 
membrane at the base of the appendages, has become 
stretched. Thus the gills from being situated close together 
have become separated. But we find an indication of their 
