306 ZOOLOGY. 
Cuass II.—PODOSTOMATA. 
Appendages of the cephalothoraa in the form of legs, spiny at the base - 
no antenna; brain supplying nerves to the eyes alone; nerves to the 
cephalothoracie appendages sent off from an wsophageal ring ; nervous 
system ensheathed by a ventral system of arteries ; metamorphosis slight. 
Sexes distinct. 
Order 1. Merostomata.—No distinct thoracic segments and appendages, 
(Limulus, Eurypterus.) 
Order 2, Trilobita.—Numerous free thoracic segments and jointed ap- 
pendages. (Agnostus, Paradoxides, Calymene, Trinucleus, 
Asaphus; all extinct.) 
CLASSIFICATION OF THE ORDERS OF CRUSTACEA AND PODOSTOMATA. 
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ice 
CRUSTACEA. PopostTomMaTa. 
Laboratory Work.—In dissecting the lobster, the shell or crust may 
be removed by a stout knife ; the whole dorsal portion of the cephalo- 
thorax and each segment behind, including the base of the telson, 
should be removed, care being taken not to injure the brain, which lies 
just under the base of the rostrum. The hypodermis, or reddish, mem- 
branous, inner layer of the integument, should then be dissected away, 
exposing the heart, the stomach, the liver, and the large muscles of 
the abdomen. The arterial system can be injected with carmine 
