XII.] THE FRESH- WATER CRAYFISH. 123 



sents an anterior division the cepTialoihorax covered 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 appen- 

 dages 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 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. In this canal 

 there lies a flat oval plate the scaphognathite which is 

 attached to the second pair of maxillse 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 correspond- 

 ing divisions in the carapace of the Lobster ; but, in the Cray- 



