542 



ZOOLOGY 



SECT. 



like as in the Macrura. The first legs (l.l) form chelipeds often 

 of great size : the remaining legs generally end in simple claws, 

 but in the Swimming-crabs the distal segment (Fig. 428, 1) 

 in the fifth pair is flattened and forms a fin. The range of 

 variation in form, proportions, colour, markings, &c., among crabs 

 is very great (Fig. 428). 



Unlike the Decapoda, the Stomatopoda form a very small order, .comprising a 

 few genera varying from the size of a Shrimp to that of a Lobster. Squilla (Fig. 429) 

 is the best known genus. 



The abdomen (al a7) is very large in proportion to the cephalothorax, 

 and the carapace (cth), which is thin and uncalcified, leaves the last three 

 thoracic segments ( VI VIII) uncovered. The rostrum is movably articulated, 

 and covers the prostomial region, which is divided into two distinct segments, 

 the first bearing the large stalked eyes, the second the antennules. This 

 arrangement appears to support the view that the aiitemiulary region is a 



FIG. 420. 



;. 420. Squilla. al, antennule ; a2, antenna ; ala7, abdominal segments ; br, gills ; 

 cephalothorax; p, copulatory organ; pi po, pleopods; p6, uropods; VI VIII, 

 thoracic segments ; 1 S, thoracic appendages. .(From Lang's Comparative Anatomy.) 



ills ; cth. 

 free 



metamere distinct from the prostomium, but the division in question is absent 

 in the larva, and does not appear till the proper segmentation of the body is 

 established : probably it has a physiological meaning, and is connected with the 

 necessity of extreme mobility of the eyes and olfactory organs in an animal 

 which lives in a burrow with only the anterior end of the head exposed. 



The antennule (al) has three flagella ; the antenna (a2) a single flagellum and 

 a very large exopodite. The first five pairs of thoracic limbs ' (1 5) are turned 

 forwards towards the mouth, and act as maxillipedes ; the second of these 

 corresponding with the second maxillipede of Astacus is very large (2), and 

 its distal segment is turned back and articulated to the penultimate segment 

 like the blade of a pocket-knife to the handle. In this way a very efficient 

 weapon called a xuh-chela is produced, both of the segments of which are pro- 

 duced into strong spines. The remaining three thoracic 'appendages (6 S) are 

 slender legs provided with exopodites : the last of them has a styliform copu- 

 latory organ (p) developed from its proximal segment. The pleopods are large 

 and biramous : the first five (pi, p5) have gill-filaments (l>r) attached to their 

 plate-like exopodites : the sixth (2)6) form large uropods or lateral tail-lobes, 

 as in Astacus. 



