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 oi 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 Phyllopoda, 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 any 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 cephalic 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 

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