ADPENDAGES AND NERVOUS SYSTEM OF APUS CANCRIFORMIS. 355 
always coincide with one another in position, that is, are 
not exactly opposite to one another. 
The third segment of the corm supports on its ventral 
margin one filamentous endite which has forty annulations 
or thereabouts. On its dorsal margin this segment gives 
attachment to the flabellum and the bract. 
The fourth segment of the corm carries one long endite 
with fifty annulations (end*) on its ventral margin, and ter- 
minally is produced into a non-movable process from which 
two endites arise, one (end°) very long and filamentous with 
eighty annulations, the other (end*) minute and boat-shaped, 
resembling the terminal endite (end*) of other thoracic limbs. 
The peculiar shape of this terminal endite and the notch of 
the corm into which its exaxial process fits is shown in 
PE XX, fie. 7a. 
It is at once obvious that in the first thoracic limb we 
have a very special differentiation of the endites in respect 
of their filamentous jointed form and of the corm in respect 
of its four movable segments. In both these features the 
second thoracic foot is intermediate between the first foot 
and those which follow posteriorly. 
The first thoracic foot at once suggests comparison with 
some of the appendages of the Podophthalmatous Crustacea, 
as also with the limbs of Nauplius, but the discussion of the 
homologies of its parts with those of the limbs of other 
Crustacea, must be deferred to a subsequent section of this 
memoir. 
The oral appendages.—The appendages immediately fol- 
lowing the mouth constitute a natural group, which are 
distinguished by their small size and peculiar structure. I 
recognise three such pairs of appendages, a first (nearest 
the mouth), the mandibles (Pl. XX, fig. 3) ; a second, the 
maxill (figs. 4, 4a, 5); a third, the maxillipedes (fig. 6). 
Very different interpretations have been placed upon these 
parts by different writers, Zaddach regarded the two pieces 
of the maxilla (figs. 4 and 5) as two distinct maxille—a first 
and second—in which view, with the curious difference that 
he transposes erroneously the natural position of the pieces, 
Gerstaecker follows him. 
The maxillipede is described by Zaddach as a rudimentary 
thoracic foot—a view which is no doubt perfectly correct, 
though, on account of its relation to the oral series, it is 
more convenient to indicate it by the term maxillipede. 
Grube agrees with Zaddach in regarding the two pieces of 
the maxilla as two distinct maxillz, and the maxillipede as 
a rudimentary foot. Claus appears to have been the first to 
