CHAPTER XIV. 

 Cettstacea. 



Class I. Cettstacea (Lat. crusta, a crust, or external 

 shell). — The members of this class are commonly known as 

 crabs, lobsters, shrimps, prawns, king-crabs, barnacles, acorn- 

 shells, wood-Uce, etc. They are nearly allied to the succeeding 

 class of the Arachnida (spiders and scorpions), but are dis- 

 tinguished by their adaptation to a more or less purely aquatic 

 life, by having jointed appendages upon the hinder segments 

 of the body (abdomen), and by the possession of two pairs of 

 antennae. As a class, the Crustacea are distinguished by 

 being usually furnished with branchiae or respiratory organs 

 adapted for breathing air dissolved in water, by having more 

 than four pairs of legs, by having a well-developed chitinous 

 or partially calcareous "crust" or external skeleton, by the 

 fact that some of the appendages are generally so modified as 

 to act as organs of mastication, and by passing through a meta- 

 morphosis before attaining their adult condition. 



The body in a typical Crustacean is composed of twenty-one 

 (or, according to some writers, twenty) distinct segments or 

 somites, placed one behind the other. These segments are 

 distributed in three distinct divisions, known respectively as 

 the " head," the " thorax " or chest, and the " abdomen " or 

 tail, each of which is usually regarded as being composed of 

 seven segments. In very many cases, however, the fourteen 

 segments belonging to the head and chest are amalgamated 

 together into a single mass, which is termed the " cephalo- 

 thorax," thus leaving seven segments to the abdomen. It 

 will be unnecessary, however, to dwell here longer upon the 

 structure of the Crustacea, as the general morphology of 

 the class will be given at somewhat greater length in speak- 

 ing of the lobster. The classification, also, of the Crustacea 



