SHRIMPS AND PRAWNS 259 



or fin-like expansion. The next to the last segment, in many forms, 

 has appendages modified into swimming-plates, which extend on 

 each side of the telson, forming a broad, fan-like caudal extremity. 



They have, then, to correspond to the twenty segments of the 

 body, two pairs of sensory, six pairs of mouth-, and five pairs of 

 walking-appendages attached to the cephalothorax, and six pairs 

 on the abdomen. The terminal segment, or telson, is without ap- 

 pendages. The exopodite is present on the maxillipeds, but dis- 

 appears from the walking-feet in the higher forms. 



In moulting the Macrura split in the longitudinal line down the 

 back ; in the Brachyura the split occurs across the body at the 

 point between the thorax and the abdomen. 



SUBORDER MACRTTRA 

 SHRIMPS, PRAWNS, LOBSTERS, CRAWFISH, AND HERMIT-CRABS 



The characteristic features of the Macrura are an elongated body 

 with the abdomen usually extended : a carapace, somewhat cylin- 

 drical ; and the last pair of appendages of the abdomen (which 

 are attached to the next to last segment) united with the last 

 segment, or telson, to form a powerful caudal fin, used for swim- 

 ming backward. The creeping forms in moving walk forward, 

 but swim backward. 



FREE-SWIMMING FORMS : SHRIMPS AND PRAWNS 



In these animals the body is compressed and the carapace is 

 not hard. The abdomen is very large in proportion to the 

 cephalothorax, and has a peculiar bend. The rostrum is often 

 longer than the thorax. The eye-stalks, antennae, and legs some- 

 times attain extraordinary length, and the chelae (claws) are not 

 always on the first pair of legs. In some species chelae are on 

 two or three pairs of the legs. Above the antennas are expanded 

 antennal scales, which, together with the long bases of the 

 antennules and very prominent eye-stalks, make the head a broad 

 and conspicuous feature. The difference between shrimps and 

 prawns is not very well defined, the small individuals seeming to 

 be generally called shrimps, the larger full-grown ones prawns. 



