Page:Walcott Cambrian Geology and Paleontology II.djvu/241

 of the Branchiopoda and Malacostraca is six if we consider the stalked eyes as representing the first pair.

The stalked eyes are distinctly shown for Opabinia (pl. 28, fig. 1), Waptia (pl. 27, fig. 4), and Yohoia (pl. 29, fig. 9), and for Hymenocaris by specimens not illustrated. Burgessia (pl. 27) and Marrella (pl. 25, figs. 4 and 5) have sessile eyes and five pairs of cephalic appendages. The sessile eyes, as in the trilobite, probably represent a segment of the cephalic shield.

The character of the several thoracic appendages is described under each species. So far as determined, the stalked eyes, antennules, and antennæ are not very unlike those of recent crustaceans of the same orders, and the mandible, maxillula, and maxilla also have the same fundamental structure with modifications to meet the needs of each genus and species.

The thoracic appendages appear to be based on the typical crustacean limb having a protopodite bearing an exopodite and endopodite. There are no recognized modifications of this that would indicate a simpler form. An epipodote (gill) is attached to the protopodite in Marrella (pl. 26, fig. 4), Opabinia (pl. 27, fig. 6), and Malaria (pl. 29, fig. 3).

The number of thoracic appendages is indicated in the table (p. 158) which gives the number of thoracic segments. Each of these thoracic segments is considered to have had a pair of attached appendages although, as in the case of Apus, the posterior segments may possibly have had more than one pair of appendages.

Simple, bifid abdominal appendages only appear on the abdominal segments of Anomalocaris (pl. 34, fig. 3). The caudal rami of the abdominal segment vary greatly in form and structure in the crustaceans from the Burgess shale. The female of Opabinia appears to have two unsegmented, expanded rami. Waptia (pl. 27, figs. 4 and 5) and Yohoia (pl. 29, figs. 8, 11, and 14) have two expanded rami with rudimentary segmentation. Hymenocaris (pl. 31, figs. 3 and 5) and Odaraia (pl. 34, fig. 2) have several cercopods attached to the last abdominal segment, and the trilobite Neolenus (pl. 24, figs. 1 and 1a) has two long, slender, jointed rami.