452 
FLORENCE BUCHANAN. 
no special respiratory organs, but the vessels of the vascular 
system were distributed equally on the whole surface of the 
body, thus enabling the liquid they contained to absorb 
oxygen through the thin wall separating them from the water 
in which the creature lived. This vascular surface then be- 
came concentrated, and it would naturally be concentrated to 
those parts of the body which are most brought in contact 
with fresh supplies of oxygen, and, consequently, of water. 
Thus when certain limbs became especially modified for swim- 
ming, it was the parts of the body-surface behind those limbs 
that first became especially vascular and branchial. It was then 
of advantage to the animal to have this vascular surface in- 
creased ; the skin therefore, at or on the base of the swimming 
appendage, became folded, and we find it thus as a simple plate 
in the nearest living representatives of the Crustacean ancestor, 
namely, the Phyllopods, as exemplified by Apus. The typical 
Apus thoracic appendage (PI. XXXVII, fig. 1) consists of a 
basal or axial portion (protopodite), sometimes divided into 
two, three, or even four parts, of six “ endites,” and of two 
<e exites,” viz. a flabellum and a bract. Of these the fifth 
endite probably represents the endopodite of the Crayfish limb ; 
the sixth represents the exopodite; the flabellum or large 
swimming plate the epipodite ; and then we have immediately 
behind the swimming plate a flattened fold of the skin or bract, 
shown by Lankester to be devoid of muscles and by Claus to 
be of different constituency to the rest of the limb by the 
rapidity with which it stains with dilute osmic acid. It is this 
that is, it seems probable, homologous with the branchiae of the 
Decapod. Anyway this vascular respiratory outgrowth is quite 
independent of what corresponds to the epipodite in the Cray- 
fish or Lobster (see fig. 9). It is worthy of note that in 
the oostigite, where the flabellum is especially modified for 
carrying the eggs, and therefore not used for swimming, the 
bract is very rudimentary. Whether it is vestigial (i. e. the 
remains of a bract that was once well developed) or rudi- 
mentary (i. e. that it never was more developed), depends, of 
course, on whether the special modification of the oosti- 
