128 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. The anus is 

 situated on the ventral aspect, beneath the telson and behind 

 the last somite. 



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

 the cervical suture, separates an anterior division, which 

 belongs to the head or cephalon, from a posterior division 

 which covers the thorax ; and the thoracic division of the 

 carapace further presents a central region, which covers 

 the head, and wide lateral prolongations, which pass down- 

 wards and cover the sides of the thorax, their free ven- 

 tral edges being applied against the bases of the thoracic 

 limbs. These are the branchiostcgitcs. 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. 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 may be observed. There is no trace of any 



