CLASS I CRUSTACEA 607 
the second and third segments, and is frequently strengthened by deposits of 
carbonate and phosphate of lime. Although the carapace is usually a single 
piece, yet in some forms (Estheriiform Phyllopods 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 Trilobites, but in the MJalacostraca they are almost constantly 
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 (4pus, Oniscids) one cr 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 Phyllopods, but the typical foot 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 talue 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 sub-classes are recognised :—Tvilobita and Eucrustacea. The term 
Entomostraca is here used in a collective sense to distinguish the lower orders 
of Eucrustacea from the highest, or JJalacostraca. 
Sub-Class 1. TRILOBITA. Trilobites.! 
Marine Crustacea, with a variable number of metameres ; body covered with a 
hard dorsal shield or crust, longitudinally trilobate from the defined axis and pleura ; 
1 Literature: A. General Works. 
Brongniart, A., Histoire naturelle des Crustacés fossiles. 1882.—Dalman, J. W., Ueber die 
Palaeaden oder die sogenannten 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, #., Ueber einige bohmische Trilobiten. Berlin, 1845-46.—Corda, 
J.C, and Hawle, J., Prodrom einer Monographie der béhmischen Trilobiten. Prag. 1847.—Hadl, 
J., Palaeontology of New York, vols. I.-If. 1847-59.—Barrande, J., Systeme Silurien du 
