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. Palaeontology, 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 Trilobita 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 some forms of _Apus_ the second pair of antennae may be rudimentary or even absent. There are still other features which characterise the Trilobita: thus the eyes are borne on free cheeks, and differ in structure from those of Phyllopods. The broad pygidium formed of fused segments and without terminal fulcra is quite unlike the slender-jointed abdomen of _Apus_ and _Branchipus_; and whilst in the Trilobites all the segments bear appendages, in the Phyllopods some, at any rate, of the posterior segments are devoid of appendages. The distinct division of the body into an axial and pleural region is not seen in Phyllopods, and is probably a character of some importance, since it occurs in the great majority of Trilobites, including all the early forms.