VII AFFINITIES 241 
on the dorsal surface, and as the free cheeks grow the ‘eyes 
move inwards and backwards, and gradually swing round 
until their long axes become parallel with the axis of the 
body. 
The larval form of Acidaspis (Fig. 145, F) is of interest since 
even in the earliest stage it shows the spiny character which 
forms such a striking feature of the adult (Fig. 151, F). 
Before the discovery of the ventral surface of Trilobites 
it was thought by some zoologists that their affinities were 
with the Xiphosura rather than with the Crustacea. But the 
presence of antennae, and of five pairs of cephalic appendages ; 
the biramous thoracic and pygidial appendages, the hypostome, 
and the character of the larval form, as well as the absence of 
a genital operculum, separate the Trilobites from the Xiphosura 
and connect them with the Crustacea. 
The position of the Trilobites in the Crustacea is, however, 
difficult to determine. Already in the Cambrian period, at least 
five main groups of the Crustacea were clearly differentiated, 
namely, the Phylopoda, Ostracoda, Cirripedia, Trilobita, and 
Leptostraca (Phyllocarida), and probably also the Copepoda, but 
of the last no remains have been preserved as fossils. Palaeon- 
tology, therefore, furnishes no connecting links between ayy two 
of these orders. 
The Crustacea to which the Trilobites show some resemblance 
are the families Apodidae and Branchipodidae of the Order 
Phyllopoda (see pp. 19-36). The Trilobita agree with those 
families in having a large but variable number of trunk- 
segments, in the possession of a large labrum (hypostome), and 
in the occurrence of gnathobases on the thoracic appendages ; 
also the foliation of some of the trunk-appendages is somewhat 
similar. The points of difference, however, are considerable ; 
thus the cephalhe appendages are much more specialised in the 
Apodidae and Branchipodidae than in the Trilobita; in the 
latter all, with the exception of the antennae, are distinctly 
biramous, and whilst the basal joints were masticatory the 
distal parts appear to have been locomotor organs. The 
appendages of the trunk also differ considerably ; in the Trilo- 
bita all are clearly biramous, those of the thorax having a 
schizopodal form. In the possession of a single pair of 
antennae the Trilobita differ from other Crustacea; but in 
VOILE AY R 
