XT 



PHYLUM ARTHROPODA 



577 



bases of the antennules are contained in sockets or fossettes. The 

 third maxillipedes (mxp.) are broad, flat, and valve-like, not leg- 

 like as in the Macrara. The first legs (LI) form chelipeds often 

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

 but in the Swimming-crabs (Fig. 476, 1) the distal segment 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. 476). 



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. 477) is the best known genus. 



The abdomen (a : a 7 ) 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 anterior head- 



FIQ. 477. Squilla. ai, antennule ; 02, antenna ; ai a/, abdominal segments ; br, gills ; cth, 

 cephalpthorax ; p, copulatory organ ; p\ p 5 , pleopods ; p 6> uropods ; VI VIII, free 

 thoracic segments ; 1 8, thoracic appendages. (Prom Lang's Comparative Anatomy.) 



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 antennulary region is a 

 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 physio- 

 logical 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 (a t ) has three flagella ; the antenna (a 2 ) a single 

 flagellum and a very large exopodite. The first five pairs of 

 thoracic limbs (7 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 



