APPENDAGES AND NERVOUS SYSTEM OF APUS CANCRIFORMIS, 363 
used) or the Crustacean limb is borne in mind, and the un- 
deniable tendency of one limb to become a pseudomorph of 
another, attaining stmilar form by identical modelling of 
parts not identical in origin, we have to be very careful in 
insisting upon conclusions arrived at by giving value to 
simple correspondence in form. 
At the same time it appears most probable, and has been 
admitted by Claus and others, that the Nauplius appendages 
of Phyllopoda are devoid of flabellum and bract. Hence we 
may formulate the conclusion that the exopodite and endo- 
podite of the Nauplius limb are homologous with the sixth 
and the fifth endites respectively of the thoracic limbs, and, 
similarly, that the flabellum is not homologous with the 
exopodite. 
But when we come to examine the Nauplius of Decapoda 
(Penzeus, &c.) we find that the exopodite of the second pair 
of Nauplian appendages becomes the short exopodite of the 
adult antennule, whilst the endopodite becomes the polyar- 
throus filament. Accordingly, if the exopodite and endo- 
podite of the antenna of such a Decapod as Astacus are cor- 
rectly identified with the parts called exopodite and endopo- 
dite (external and internal ramus) in the postoral series, 
the flabellum cannot be homologous with the exopodite of the 
maxillipedes and maxille. Such an homology might, however, 
be maintained by abandoning the current identification of 
the parts of the preoral and postoral appendages of Astacus. 
With regard to the antennules or first preorals of Astacus, 
there seems to be but little to justify the homology implied 
in calling its two rami respectively exopodite and endopodite, 
since in no Nauplius is this appendage biramose. Its distal 
segment in the Nauplius phase may with probability be iden- 
tified with the external ramus (endite 6) of the antenna, but 
its inner ramus appears in Penzeus at a late period, and may 
or may not be the equivalent of endite 5, the internal ramus 
of the antenna. 
The drawing XVJ in the woodcut, fig. 1, represents the 
form of all the thoracic appendages in the developing Apus 
(a younger condition is drawn in X/). It is interesting 
to note the extreme modification which the apophyses subse- 
quently undergo in the case of the first thoracic foot. The fla- 
bellum and the bract expand, but the terminal endite becomes 
almost rudimentary and claw-like, whilst endite 5 grows out 
as a long polyarthrous filament, and endite 4 attains nearly 
tbe same length; endites 8 and 2 become shorter polyar- 
throus filaments, and endite 1 becomes the jaw process. On 
a first glance at the adult limb (Pl. XX, fig. 7) one would 
