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 MACRURA 
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 caranace 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, antenne, and legs some- 
times attain extraordinary length, and the chelz (claws) are not 
always on the first pair of legs. In some species chele are on 
two or three pairs of the legs. Above the antenne 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 1s not very well defined, the small individuals seeming to 
be generally called shrimps, the larger full-grown ones prawns. 
