süBCLASs II EUCRUSTACEA— BRANCHIOPODA 731 



the opiiiioii of bheir describer may have been derived from the same general 

 type of animal as the accompanying crustaceans. 



The preceding diagram, which is taken from Walcott, illustrates that 

 author's views as to the relations of Cambrian crustaceans to a theoretical 

 ancestral stock which for convenience is correlated with ^^'ws-like forms. 

 From this primitive stock it is assumed that the Branchiopoda were derived, 

 and from them three distinct branches were developed prior to, or diiring, 

 Cambrian time. According to this view Trilobita are direct descendants of 

 Branchiopoda, and in turn gave rise to the line leading through the Orders 

 Aglaspina, Limulava and Eurypterida to the Xiphosura. The structure and 

 probable habits of Trilobites lead Walcott to the opinion that these w^re 

 mud-burrowing animals more or less allied to Limulus. The Phyllocarida 

 and Ostracoda are assumed by the same author to have been derived from 

 the Branchiopoda, but on different lines of descent from that of the Trilobites 

 and the Orders grouped under the Merostomata. 



The Eucrustacea are frequently divided into Entomostraca and Malacostraca^ 

 but the first of these groups is not a homogeneous assemblage ; it is rather a 

 repository in which have been placed all forms not members of the Malacostraca. 

 The Eucrustacea are here divided into the superorders Branchiopoda, Ostracoda^ 

 Copepoda, Cirripedia and Malacostraca. 



Superorder 1. BRANCHIOPODA Latreille.i 



Eucrustacea in which the carapace may form a dorsal shield or a bivalve shell or 

 may he entirely ahsent ; the number of body segments and appendages varies greatly ; 

 the appendages of the body are rarely pediform, generally foliaceous and lobed. 



Under the Branchiopods are embraced very differently formed Crustaceans 

 of large and small size, living mostly in fresh water or salt lakes, and 

 possessing little eise in common than the leaf-like form of leg. The 

 segmentation of the body in the larger forms is very distinct, but in. the 

 water fleas (Cladocera) it is usually quite incomplete. The number of body- 

 segments varies considerably among different genera. In the strongly 

 segmented forms the body is elongated and protected in front by a flat or 

 shield-shaped dorsal carapace (Apus), or it is naked (Branchipus), In the 

 Cladocera and Estheriidae, which are enclosed in a bivalve shell, the body is 



^ Literat ure : A. Recent Forms. — Grube E.., Bemerkungen über die Phyllopoden, etc. Wieg- 

 mann's Archiv für Naturgesch., 1853-1865, vols. xix., xxi. — Claus, C, Papers ou Branchipus 

 and Apus, in Abhandl. Gesellsch. Wiss, Göttingen, 1873, vol. xviii., and Arbeit. Zool. Inst, Wien, 

 1886, vol. vi. — Weismann, F. L. A., Zur Naturgeschichte der Daphniden. Zeitschr. Wissensch. 

 Zool., 1876-80, vols. xxvii., xxxiii. — Lankester, JE. R., Several papers on Limulus, Apus, etc., 

 in Quart. Journ. Microsc. Soc, 1881, vol. xxi. — Packard, A. S., Monograph of the Phyllopod 

 Crustacea of North America, 12th Ann. Rept. U.S. Geogr. and Geol. Surv. Terr., 1883.— Äirs, G. 0., 

 Fauna Norvegiae. I. Phyllocarida and Phyllopoda, 1896. — Bernard, H. M., The Apodidae. Nature 

 series, London, 1892. 



B. Fossil Forms. — Jones, T. R., On Fossil Estheriae and their distribution. Quar. Journ. 

 Geol. Soc, 1863, vol. xix.— Monograph of the Fossil Estheriae. Palaeontogr. Soc, 1862.— 5th 

 and 7th Repts. Comm. British Assoc Adv. Sei. on Fossil Phyllopoda, 1887-89.— Geol. Mag., 

 Sept. 1890, Feb. 1891, Dec 1893, July 1894.— Trans. Geol. Soc. Glasgow, 1890, vol. ix.— 

 Glarke, J, M., New Devonian Phyllopods. Amer. Journ. Sei., 1882, vol. xxiii. — Hall, J., and 

 Glarke, J. M., Palaeontology of New York, 1888, vol. viu—Bemard, IL M., Fossil Apodidae. 

 Nat. Sei., 1897, vol. xx.—Schuchert, C., On the fossil Phyllopod genera Dipeltis and Protocaris. 

 Proc. U.S. Nat. Mus., 1897, vol. xix.—Clarke, J. M., Estheria in Devonian of New York and 

 Carboniferous of Ohio. Rept, N.Y. State Paleontologist, 1900.— 7</eH(, Notes on Paleozoic Crus- 

 taceans. 54th Ann. Rept. N.Y. State Mus., 1902, vol. \.— Walcott, C. D., Middle Cambrian 

 Branchiopoda, Malacostraca, etc. Smithson Mise. Coli., 1912, vol. Ivii., no. 6. 



