292 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 

 wa tery 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 student is now prepared to understand the descrip- 

 tions of the types of the different orders. The Crustacea 

 may be divided into two series of orders, or two subclasses, 

 i. e., the normal Crustacea, in large part composed of spe- 

 cies now living, though containing numerous palaeozoic forms, 

 for which we would propose the term Neocarida ; and the 

 more generalized type of Crustacea, which abounded almost 

 to the exclusion of other Crustacea, except the bivalved mi- 

 nute forms, in the Palaeozoic age of the world, and which 

 may be called the Palaocarida. They are represented by 

 the fossil Trilobites and Eurypterida, and the only surviving 

 member, the King-Crab (Limulus). 



We will now study the orders of Neocarida, treating of 

 them as heretofore in the ascending order. 



Order 1. Cirripedia. The barnacles would, at a first 

 glance, hardly be regarded as Crustacea at all, so much 

 modified is the form, owing probably to their fixed, para- 

 sitic mode of life. Indeed they were formerly placed among 

 the Mollusca, until, in 1836, Thompson found that the 

 young barnacle was like the larva of other low Crustacea 

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



