ARTHROPOD A. 137 



Crustacea, the Arachnida, the Myriapoda, and the Insecta; which 

 are roughly distinguished as follows : — 



1. Crustacea. — Respiration by means of gills, or by the general 

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

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

 usually, of the abdomen also. 



2. Arachnida. — Respiration hy pulmonary vesicles, by tracJiece, or 

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

 eephalothorax. Antennce {as such) absent. Legs eight. Abdomen 

 without articulated appendages. 



3. Myriapoda. — Respiration hy trachece. Head distinct ; re- 

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

 of antennce. Legs numerous. 



4. Insecta. — Respiration by trachece. Head, thorax, and abdomen 

 distinct. One pair of antennce. Three pairs of legs home on the 

 thorax. Abdomen destitute of limbs. Oenercdly two pairs of Vjhigs 

 on the thorax. 



CLASS I. CRUSTACEA. 



The members of this class are commonly known as Crabs, Lobsters, 

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

 They are nearly allied to the succeeding class of the Arachnida 

 (Spiders and Scorpions), but are distinguished 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 pos- 

 session of two pairs of antennae. As a class, the Crustacea are distin- 

 guished 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 metamorphosis before 

 attaining their adult condition. 



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

 tinct segments or somites, placed 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 thorax are amalgamated together into a 

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

 segments to the abdomen. 



Each segment or " somite " (fig. 86) may be regarded as com- 



