692 AKTHROPODA phylum vii 



piece, yet in some forms (Estheriiform Branchiopods and Ostracods) it may 

 consist of two lateral valves, which enclose the body like a Pelecypod shell ; 

 or of four parts, as in certain Phyllocarida ; or again (Cirripedia) of a number 

 of calcareous plates. The abdomen is usually well developed and its seg- 

 ments are free, but occasionally it becomes greatly reduced, as in certain 

 Entomostraca. 



The total number of body somites varies within wide limits in the 

 Entomostraca and Trilobita, but in the Malacostraca they are almost constantl}^ 

 twenty -one, ranging slightly higher in the Phyllocarida, and falling shorter in 

 the parasitic Laemodipoda. ' 



In all living Crustacea there are two pairs of antennae, although in some 

 forms {Apus, Oniscids) one or the other pair may become greatly reduced. 

 In the Trilobites, on the other hand, but a single pair has been discovered. 

 The appendages are exceedingly variable in form, according as they serve 

 for sensation, comminution of food (" mouth parts "), locomotion, respiration, 

 capture of prey or copulation. The primitive form was a lamellar appendage 

 like those found in the thoracic region of Branchioj)ods, but the typical leg is 

 usually stated to consist of a basal portion (protopodite) of one or two joints, 

 and a distal portion made up of an inner (endopodite) and a lateral branch 

 (exopodite). In many cases the exopodite becomes greatly modified or even 

 entirely atrophied in the adult. 



Most of the lower Crustacea escape from the egg in a larval condition 

 known as the nauplius stage. In the nauplius the body is unsegmented, there 

 is but a single median eye, and but three pairs of appendages, correspond- 

 ing to the two pairs of antennae and mandibles of the adult. The nauplius 

 gradually becomes metamorphosed into the adult Crustacean, the changes being 

 accomplished by several moults of the external chitinous crust. In the higher 

 Crustacea this free-swimming nauplius stage is omitted, the animal already 

 having the form of the adult as it escapes from the egg. The Decapods have 

 a larval stage known as the zoea, in which seven pairs of appendages and a 

 segmented abdomen are present. These larval stages are of great value in 

 determining relationships, but most modern authorities regard them as adaptive 

 rather than ancestral ; or, in other words, it is not believed that existing 

 Crustacea are descended from an ancestral form resembling the nauplius. 



Two subclasses are recognised : Trilobita and Eucrustacea. The term 

 Entomostraca is here used in a collective sense to distinguish the lower orders 

 of Eucrustacea from the highest, or Malacostraca. 



Subclass A. TRILOBITA Walch. Trilobites.' 



Marine Crustacea, with a variable number of metameres ; body covered with a 

 hard dorsal shield or crust, longitudinally trilobate into the defined axis ami pleura ; 



^ Litei-ature : A. General Works. — Brongniart, A., Histolre naturelle des Crustacus fossiles. 1822. 

 — Dalmicn, J. IF., Ueber die Palaeadeii oder die .sogeiiannten Trilobiten. 1828. — Green, J., 

 Monograph of the Trilobites of North America, with coloured models of the species. 1832. — 

 Burmeister, H., Die Organisation der Trilobiten. 1843. — Beyrich, E., tjber einige biihniische 

 Trilobiten, Berlin, 184.'J-46. — Cort^a, J. C, and Jlmvle, J., Prodrora einer Monogi-aphie der 

 liohmischen Trilobiten. Prag 1847. — IIccU, J., Palaeontology of New York, vols, i.-iii., 1847-59. — 

 Barmnde, J., Systunie Hilurien du centre de la BohOnie, vol. i., 1852 ; Supplement, 1872. — Angelin, 

 N. P., Palaeontologia Scaudinavica. Parti. Crustacea formationis transitionis, 1854. — Neiszkuivski, 

 J., Versuch einer Monographic der in den silurischen Sehichten der Ostseeprovinzeii vorkoninii'inlen 

 Trilol)iten, 1857. — Hoffmann, E., Siinmitliche bisjetzt bekaunte Trilobiten Ivusslands. Vcrhandl. 



