200 MANUAL OF ZOOLOGY. 



tudinal commissures, and traversed anteriorly by the oesophagus. 

 The haemal system, when differentiated, is placed dorsally, and 

 consists of a contractile cavity, or heart, provided with valvular 

 apertures, and communicating with a perivisceral cavity, con- 

 taining corpusculated blood. Respiration is effected by the 

 general surface of the body, by gills, by pulmonary sacs, or by 

 tubular involutions of the integument, termed "tracheae." In 

 no member of the division are vibratile cilia known to be 

 developed. According to Professor Huxley, an additional 

 constant character of the Arthropoda is to be found in the 

 structure of the head, which is typically composed of six seg- 

 ments, and never contains less than four. 



The Arthropoda are divided into four great classes — viz., the 

 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 antenncB. Locomotive appen- 

 dages more than eight in number, borne by the segments of the 

 thorax, and usually of the abdomen also. 



2. Arachnida. — Respiration by pulmonary vesicles, by trachecB, 

 or by the general surface of the body. Head and thorax united 

 into a cephalothorax. Antennce {as such) absent. Legs eight. 

 Abdomen without articulated appendages. 



3. Myriapoda. — Respiration by 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, 

 borne on the thorax. Abdomen destitute of limbs. Generally two 

 pairs of wings on the thorax. 



CHAPTER XXXI. 

 CRUSTACEA. 



Class I. Crustacea. — The members of this class are com- 

 monly known as Crabs, Lobsters, Shrimps, King-crabs, Bar- 

 nacles, Acorn-shells, &c. They are nearly allied to the suc- 

 ceeding order of the Arachnida (Spiders and Scorpions) ; but 

 may usually be distinguished by the possession of articulated 

 appendages upon the abdominal segments, by the possession 

 of two pairs of antennse, and by the presence of branchiEe. 



