114 CRUSTACKANS. 



Mild that these appendages vary greatly in si/e on I lie 

 different segments. 



I. Cephalothorax. - - The hard shell, which entirely 

 -(\ci's I li- cephalothorax in its normal position, is tlie 

 carapace. 'I 'In-, stout beak extending forward from the 

 front of it, between the eyes, is the rostrum. Observe 

 that the whole is divided into portions corresponding 



l,o head ;ind thorax by a shallow n-n^'rn/ //rm>/v lha! runs 



transversely across the top of the carapace, and extends 

 obliquely l'or\v;ml along the sides. The two longitudinal 

 grooves on the carapace hack of t he cervical groove, which 

 mark the line of separation of the two gill chambers from 

 the thorax proper, have been noticed already. 



II. Abdominal Segments. Note the number and form 

 of the abdominal segments. Flex and extend the abdomen, 

 and study their action. Take the third or fourth abdomi- 

 nal segment as a type, and in it locate the parts as named. 

 The convex dorsal plate of the segment is the notwm, or 

 teryum ; the projecting plate that hangs down on either 

 Hide like the eaves of a roof is (he //A-///-/////.; I he ventral 

 transverse bar between the bases of the two appendages is 



the sternum. The appendages themselves are jointed, and 

 two-parted at the tip. 



Observe the soft membranous parts connecting the ab- 

 dominal segments, and the pivotal joining of the segments 

 together at their lateral edges, facilitating the ilexion 

 and extension of the abdomen, but preventing lateral 

 motion. 



At the posterior end of the body is the tail fin, and its 

 central piece is the terminal segment of the body, and the 

 seventh abdominal segment, called the telson. It bears no 

 appendages, and has but slight external resemblance to 

 the other body segments, being much modified to suit a 

 special purpose. The two pairs of similar broad flaps, 

 which, together with the telson, comprise the tail fin, are 



