272 ZOOLOGY. 



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, oesophagus, 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 lamellae 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 

 Palaeozoic age. In the Cambrian period also flourished 

 Ostracods, while barnacles date from the Upper Silurian 

 period. The oldest Phyllopod Crustacean is an Estheria of 

 the Devonian period, at which time also appeared the first 

 shrimp. In the Carboniferous period appeared the Gam- 

 psonychidce, a family of Schizopod shrimps, represented in 

 the United States by Palwocaris typus; also a family of true 

 shrimps, the Anthracaridce, represented by Antlirapcdcemon. 

 During this period also lived the Syncarida, a group connect- 

 ing the sessile-eyed and stalk-eyed Crustacea, *'.., 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, atafirst 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 larvse 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. 



