Page:Walcott Cambrian Geology and Paleontology II.djvu/242

 Most of the Branchiopoda are provided with strong, broad, setiferous swimming exopodites that probably also served in Marrella (pls. 25 and 26), Opabinia (pl. 27, fig. 6, and pl. 28, fig. 1), and Leanchoilia (pl. 31, fig. 6) to bring food to the mouth.

In a future paper I expect to illustrate and describe in detail the appendages of each species so far as the material will permit.

Alimentary canal.—This has been preserved in a number of species. In Opabinia (pl. 28, fig. 1), Marrella (pl. 26, fig. 6), and Burgessia (pl. 27, figs. 1 and 2), it is straight from the head to its posterior end and expands more or less in the cephalic region. The size of the canal varies from the head to the anus as in Opabinia (pl. 28, fig. 1) and Marrella (pl. 25, fig. 6 and pl. 26, fig. 6) but how much this may be due to flattening in the shale is uncertain. In Burgessia (pl. 27, figs. 1 and 2) the canal is large at the point where the hepatic tubes join it, and tapers to its posterior end. It is rounded as though retaining its contents in a fossil state. This is also true of the slender rounded canal of Hymenocaris (pl. 31, fig. 2). In Burgessia the hepatic tubes enter it back of the maxillæ. The stomach is indicated by the expansion of the anterior end of the alimentary canal. It is also outlined by a slight contraction of the canal (pl. 27, fig. 1).

Hepatic cæca.—The hepatic cæca are beautifully preserved in the shield of Burgessia (pl. 27, figs. 1-3), Naraoia (pl. 28, fig. 4), and Molaria (pl. 29, fig. 3). In Burgessia they reach their greatest development, the branches showing in fine detail on the dark shale. No definite structure has been detected in the dorsal spines of Marrella (pl. 26, fig. 1) that could certainly be referred to as the glands, but the fact that the spines have a relatively large central canal suggests that they may have contained them.

Among recent crustaceans the hepatic cæca are branched in some copepods, Corycæidæ and Asterocheridæ, but none have the beautiful structure found in Burgessia. We called the latter the "Kidney crab" in camp on account of the shape of the cæca, but as the cæca open directly into the alimentary canal they could hardly function as kidneys.

The Cambrian crustacean fauna suggests that five (p. 156) main lines or stems (Branchiopoda, Malacostraca, Ostracoda, Trilobita, and Merostomata) were in existence at the beginning of Cambrian time and that all of them had already had their