B! NTOMOSTRACA. TRILOB1TA. 361 



obtaining in the Malacostraca.* The median eye of the larva 

 nearly always persists, though often with compound lateral eyes 

 in addition. The stomach is usually without a masticatory 

 apparatus, though a regular " mill " is present in some Ostracods, 

 notably in Bairdia. The ganglion of the second antennary 

 segment retains its suboesophageal position at any rate in two 

 divisions of the Branchiopods. Among Phyllopods also we find 

 a very simple condition of the heart, it being (in BrancMpus) a 

 uniform tube with a pair of ostia corresponding to each of the 

 segments in which it lies. 



On the whole the Entomostraca are distinguished by a simple 

 and apparently more primitive grade of organization than is 

 found in the Malacostraca, and also by the absence of those 

 characters by the possession of which the Malacostraca are 

 united. They are however a much less homogeneous group than 

 the latter, and it is the fact that most divisions of the Entomos- 

 traca differ more from one another than Nebalia does from some 

 members of the Phyllopoda. 



Each group which has left any palaeontological record at all 

 was already well differentiated by the Ordovician period. The 

 Trilobites died out early, but certain genera of the Branchiopoda, 

 Ostracoda and Cirripedia have persisted from Ordovician times 

 to the present day. 



Order 1. TRILOBITA. f 



Pa 7 aeozoic Crustacea with one pair of antennae and (apparently) 

 four other pairs of cephalic appendages, the gnathobases of the 

 latter serving as jaws. Of the numerous (up to 30} segments of 

 the trunk the anterior are free and the posterior are united into a 

 pygidium. All the trunk segments except the last (telson) bear 

 biramous appendages. 



* In the Ostracoda (Cypris) however both antennary and maxillary 

 glands are found by Glaus to be present in the adult. 



f Burmeister, Die Organisation der Trilobiten, etc., Berlin, 1843. 

 Beyrich, Unters. ub. Trilobiten, Berlin, 1845, 1846. Barrande, Systeme 

 silurien du centre de la Boheme, Prague, 1852. Salter, S. W., A mono- 

 graph of the British Trilobites, London, 1864-1866. Walcott, C. D., 

 Fossils of the Utica Slate, Trans. Albany Inst., vol. X, 1883 (separate 

 copies 1879). Id., Note on some appendages of Trilobites, Proc. Biol. 

 Soc. Washington, vol. 9. Beecher, C. E., Several papers (in American 

 Journ. of Sc., ser. Ill, vol. 46 and 47, and in American Geologist, vols. 13, 

 15 and 16), collected in Studies in Evolution, Yale Univ. Publ., London, 

 1901. 



