272 ZOOLUG Y. 
rostrum or beak, where it is too solid to separate. The lobster 
then draws its body out of the rent in the anterior part of 
the carapace. The claw—at this time soft, fleshy, and very 
watery—is drawn out through the basal joint, without any 
split in the old crust. In moulting, the stomach, with the 
solid teeth in the cardiac portion, is cast off with the old in- 
tegument ; why the stomach can thus be rejected is explained 
by the fact that the mouth, esophagus, and stomach are con- 
tinuous in early embryonic life with the epithelium forming 
the outer germ-layer, the mouth and anterior part of the 
alimentary canal being the result of an invagination of the 
ectoderm. The old skin is originally loosened and pushed 
away from the hypodermis, or under-layer, by the growth of 
temporary stiff hairs, which disappear after the skin is cast ; 
the hairs, however, at least in the craw-fish, do not occur on 
the line of the facetted cornea, on the eye-stalk, or on the 
inner lamelle of the fold of the carapace over the gill- 
opening. 
The Crustacea first appeared, so far as the geological record 
shows, during the Cambrian period, as the remains of a Hy- 
menocaris occur in the Lingula flags with those of Trilobites. 
This is a Phyllocaridan, an order which characterizes the 
Paleozoic age. In the Cambrian period also flourished 
Ostracods, while barnacles date from the Upper Silurian 
period. The oldest Phyllopod Crustacean is an Hstheria of 
the Devonian period, at which time also appeared the first 
shrimp. In the Carboniferous period appeared the Gam- 
psonychide, a family of Schizopod shrimps, represented in 
the United States by Palwocaris typus; also a family of true 
shrimps, the Anthracaride, represented by Anthrapalemon. 
During this period also lived the Syncarida, a group connect- 
ing the sessile-eyed and stalk-eyed Crustacea, 7.¢., the Iso- 
pods and Decapods. The Isopods appeared in the Devonian 
period, while the genuine crabs appeared in the Jurassic period. 
Order 1. Cirripedia.*—The barnacles would, ata first glance, 
hardly be regarded as Crustacea at all, and were regarded 
as Mollusca, until, in 1836, Thompson found that the 
young barnacle was like the larve of other low Crustacea 
(Copepoda). The barnacle is, as in the common sessile form 
* The Phyllopoda are perhaps the earliest, most generalized group. 
See p. 305. 
