272 ZOOLUGT. 



rostrum or beak, where it is too solid to separate. The lobster 

 then draws its body out of the reat 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 month, 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 hypodej-mis, or under-layer, by ttie 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 Ir.mcllae 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- 

 menocar is occur in the Liiigula flags with those of Trilobites. 

 This is a Phyllocaridan, an order whicli characterizes the 

 Palseoziiic age. In the Cambrian period also flouiished 

 Ostracods, while barnacles date from the TJ)>per Silurian 

 period. The oldest Phyllopod Crustacean is an Estheria of 

 the Devonian period, at which time also appeared the first 

 shrimp. In the C^irboniferous period appeared the Gam- 

 psonychidcB, a family of Schizopod shrimps, represented in 

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

 shrimps, the Aiithracaridw. represented by AnfhrapaJcemoii. 

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

 ing the sessile-eyed and stalk-eyed Crustacea, i.e., the Iso- 

 pods and Decapods. The Isopods appeared in the Devonian 

 period, while thegenuinecrabsappeared in the Jurassic period. 



Order 1. Cirripedia. — Thebarnacles would, atafirst glance, 

 hardly be regarded as Crustacea at all, and were regarded 

 as Mollusca, until, in l!>36, Thompson found that the 

 young barnacle was like the larvse of other low Crustacea 

 {Copepoda). The barnacle is, as in the common sessile form 



