AKTHUOPOUA. 137 



Crustacea, the Araclinida, the Mijruipoda, and the Iiisecta ; which 

 are roughly distinguished as follows : — 



1. Crustacea. — Respiration ht] means of gills, or hy the general 

 surface of the body. Two pairs of antennie. Locomotive appendages 

 more than eight in number, home hy tlie segments of the thorax, and, 

 usually, of the ahdomen also. 



2. Arachnida. — Respiration hy p^dnionary vesicles, hy traeJieee, or 

 by the general surface of the body. Head and thorax united into a 

 cephalothorax. Antennae (as such) absent. Legs eight. Ahdomen 

 without articulated, appendages. 



3. Myriapoda. — Respiration by trachea!. Head clistinxi ; re- 

 mainder of the body composed of nearly similar somites. One pair 

 of antennce. Legs numerous. 



4. Insecta. — Respiration by trachae. Head, tJiora.r, and ahdomen 

 distinct. One pair of antennce. Tiiree ixxirs of legs borne on the 

 thorav. Abdomen destitute of limbs. Generally tioo pairs of icings 

 on the thurax. 



CLASS I. CRUSTACEA. 



The members of this class are commonly known as C'rabs, Lobsters, 

 Shrimps, Prawns, King-crabs, Barnacles, Acorn-shells, Wood-lice, &c. 

 They are nearly allied to the succeeding class of the Araclinida 

 (Spiders and Scorpions), but are distinguished by their adaptation to 

 a more or less pui'ely aquatic Ufe, by having jointed appendages 

 upon the liiuder segments of the body (abdomen), and by tlie pos- 

 session of two pairs of autenn;ie. Asa class, tlie Crustacea are distin- 

 guished by being usnall_v furnislied witli branclii;ie, or respiratory 

 organs a,da]ited for brcatliing air dissolved in water, by having 

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

 or partially calcareous " crust " or external skeleton, Ijy the fact that 

 some of the appendages are generally so modified as to act as organs 

 of mastication, and by passing through a metauiorphosis before 

 attaining their adult condition. 



The body in a tyjiical Crustacean is composed of ticenty-one dis- 

 tinct segments or somites, jjlaced one behind the other. These seg- 

 ments are distributed in three distinct divisions, known respec- 

 tively as the "head," the "thorax" 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 tliorax aie amalgamated together into a 

 single mass, which is termed the " cejihalothorax," thus leaving seven 

 segments to the abdomen. 



Each segment or "somite" (tig. 8(i) may lie regarded as com- 



