174 ELEMENTARY BIOLOGY. [CHAP. 



by a large continuous shield, or carapace ; and a posterior 

 division — the abdomen — divided into a series of segments 

 which are moveable upon one another in the direction of 

 the vertical median plane, so that the abdomen can be 

 straightened out, in extension ; or bent into a sharp curve, in 

 flexion. Of these segments there are seven. The anterior 

 six are the somites of the abdomen, and each of them has a 

 pair of appendages attached to its ventral wall. The seventh 

 bears no appendages and is termed the telson — it is sub- 

 divided into two pieces in the Crayfish. The anus is 

 situated on the ventral aspect, beneath the telson and behind 

 the last limb-bearing somite. 



A groove on the surface of the carapace, which is termed 

 the cervical suture, separates an anterior division, which 

 is termed the head or cephalon, from a posterior division 

 or thorax; and the thoracic division of the carapace fur- 

 ther presents wide, lateral prolongations, which pass down- 

 wards and cover the sides of the thorax, their free ventral 

 edges being applied against the bases of the thoracic 

 limbs. These are the branchiostegites. Each roofs over a 

 wide chamber in which the gills are contained and which 

 communicates with the exterior, below and behind, by the 

 narrow interval between the edge of the branchiostegite and 

 the limbs. Anteriorly and inferiorly, the branchial chamber 

 is prolonged into a canal, which opens in front and below 

 at the junction of the head with the thorax, immediately 

 behind the cervical suture. In this canal there lies a flat 

 oval plate — the scaphognathite— which is attached to the 

 second pair of maxillae and which plays a very important 

 part in the performance of the function of respiration. Of 

 the thoracic limbs themselves there are eight pairs, and, 

 on the ventral face of the body, the lines of demarcation 

 between the eight somites to which these limbs belong 



