Sao hirsuta is a species found in the Cambrian, the development of which was fully described by Barrande. Its earliest protaspis stage (Fig. [143], A) is circular in outline; the glabella expands in front and reaches the anterior margin; the pygidial region is not distinctly separated from the cephalic region; segmentation is indicated in the former, and the neck-ring is present in the latter; the eye-line is seen on each side of the glabella near the anterior margin. In a later stage (Fig. [143], C) the segmentation of the glabella becomes more distinct, indicating the existence of five cephalic segments, and the facial suture appears near the margin limiting a very narrow free cheek. Subsequently (Fig. [143], D-F) the thoracic segments develop, and increase in number until the adult stage (G) is reached; also the eyes appear at the margin of the cephalic shield, and gradually move inwards, and the glabella becomes narrower and rounded in front, and ceases to reach the anterior margin. In this species the eye-line is present in the adult.
Fig. [144].—Triarthrus becki, Green. Ordovician. A, B, Two successive stages of the protaspis, × 45. (After Beecher.)
In the protaspis of Triarthrus (Fig. [144]), found in the Ordovician, the glabella does not reach the front margin nor expand in front as it does in Sao; an eye-line is present, but disappears before the adult stage is reached.
Fig. [145].—Larval stages of Trilobites. A-D, Dalmanites socialis, Barr. Ordovician, Bohemia. The small figures below show the natural size of each specimen. (After Barrande.) E, Mesonacis asaphoides, Emmons, × 10. Lower Cambrian, North America. (After Walcott.) F, Acidaspis tuberculata, Conrad, × 20. Lower Helderberg Group (Lower Devonian or Upper Silurian), Albany County. (After Beecher.)
Dalmanites (Fig. [151], C) is a more advanced type than Sao and Triarthrus, and is found in later deposits. In the earliest stage (Fig. [145], A) the head and pygidium are quite distinct, and there is no eye-line present at this or any stage in development, but large ovoid eyes are found on the front margin, and have their long axes placed transversely to the axis of the body; the glabella is strongly segmented and is rounded in front. In later stages (C, D) the pygidium increases in size relatively, and the thoracic segments are successively introduced; the facial sutures and free cheeks appear 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 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.