APPENDAGES AND NERYOUS SYSTEM OF APUS CANCRIFORMIS, 357 
A curious feature about these appendages is, that they 
resemble bits of a thoracic foot rather than a whole thoracic 
foot in a rudimentary condition. The corm or axis is utterly 
evanescent, a mere ridge (Pl. XX, fig. 6, av). To this are 
attached two very well developed processes, one of which 
(Br) is clearly enough the bract, being identical in every 
respect with the bracts of the post-genital thoracic limbs. 
The other process is an oval chitinous plate, with long mar- 
ginal setze (en!) ; it may possibly represent the flabellum, but 
more probably one of the endites, perhaps endite 1 (the 
gnathobase). There is no means of deciding this point, for 
Claus gives but a very slight allusion to the early condition 
of this appendage in his account of the development of 
Apus. 
A highly important fact relative to this appendage was 
pointed out by Zaddach. Whilst a separate ganglion-pair 
of the ventral nerve-cord supplies the mandibles with nerves, 
and another distinct ganglion pair supplies the maxille, and 
another supplies the first thoracic foot, and a distinct gan- 
glion pair is present for each of the succeeding pairs of 
thoracic feet, this pair of appendages has no ganglion pair 
appropriated to it, but receives its nerves from the longitudinal 
cord connecting the maxilla’s ganglion pair with that of the 
first thoracic foot (woodcut, fig. 2, V mp.). It is impor- 
tant to observe that there is no evidence here of the fusion 
of two ganglion pairs and the consequent supply of two pairs 
of appendages from one and the same ganglion, but the 
nerves to the maxillipedes come off from the longitudinal 
commissure and not from a ganglion at all. In fact, we 
have evidence of the total disappearance of a ganglion pair 
corresponding to these rudimentary limbs. 
Any facts tending to establish the occurrence of interca- 
lation or of excalation of a segment (as represented by such 
structures asnerve ganglion-pairs and appendages) in the series 
building up the body of a Crustacean, Insect, or Arachnid is of 
great interest, since, if it be once admitted that a segment 
may disappear, or that a new segment may be introduced, 
the attempt to derive the series of segments and appendages 
of an Insect from those of a Crustacean or of an Arachnid, 
or of all three from a common ancestral form, loses all the 
difficulty which is encountered when the disappearance of 
segments is admitted as occurring only in the extreme poste- 
rior region, or only in that and the extreme anterior region 
of the series. 
The maxillipedes of Apus appear to be rudimentary 
structures in the sense of structures which are degenerating 
