l6o SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



Most of the Branchiopoda are provided with strong, broad, setif- 

 erous svi^imming exopodites that probably also served in Marrella 

 (pis. 25 and .26), Opabinia (pi. 27, fig. 6, and pi. 28, fig. i), and 

 Leanchoilia (pi. 31, fig. 6) to bring food to the mouth. 



In a future paper 1 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 (pi. 28, fig. i), Marrella (pi. 26, fig. 6), and Burgessia 

 (pi. 27, figs. I 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 (pi. 28, fig. i) 

 and Marrella (pi. 25, fig. 6 and pi. 26, fig. 6) but how much this may 

 be due to flattening in the shale is uncertain. In Burgessia (pi. 27, 

 figs. I 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 retain- 

 ing its contents in a fossil state. This is also true of the slender 

 rounded canal of Hymenocaris (pi. 31, fig. 2). In Burgessia the 

 hepatic tubes enter it back of the maxillae. 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 (pi. 27, fig. i). 



Hepatic cceca. — The hepatic caeca are beautifully preserved in 

 the shield of Burgessia (pi. 27, figs. 1-3), Naraoia (pi. 28, fig. 4), 

 and Malaria (pi. 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 (pi. 26, fig. i) 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 caeca are branched in some 

 copepods, Corycaeidae and Asterocheridae, but none have the beauti- 

 ful structure found in Burgessia. We called the latter the " Kidney 

 crab " in camp on account of the shape of the caeca, but as the caeca 

 open directly into the alimentary canal they could hardly function as 

 kidneys. 



ORIGIN OF MIDDLE CAMBRIAN CRUSTACEAN FAUNA 



The Cambrian crustacean fauna suggests that five (p. 156) 



main lines or stems (Branchiopoda, Malacostraca, Ostracoda, Trilo- 



bita, and Merostomata) were in existence at the beginning of 



Cambrian time and that all of them had already had their incep- 



