592 



ZOOLOGY 



but in the Swimming-crabs (Fig. 469, 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. 469). 



Unlike the Decapoda, the Stomatcywda form a very small order, 

 comprising a few genera varying from the size of a Shrimp to that 

 of a Lobster. Squilla (Fig. 470) 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 uncalci- 

 fied, leaves the last three thoracic segments (VI — VIII) un- 

 covered. The rostrum is movably articulated, and covers the 

 anterior head-region, which is divided into two distinct segments, 

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



Fio. 470. — Squilla. al, antennule ; a%, antenna ; al — a7, abdominal segments ; br, gills ; cth, 

 cephalothorax ; p, copulatory organ ; pi — p.5, pleopods ; p6, uropods ; VI — VIII, free thoracic 

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



This arrangement appears to support the view that the anten- 

 nulary region is a metamere distinct from the prostornium ; 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 flagolla ; the antenna (a8) 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 seg- 

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

 a very efficient weapon called a sub-chela is produced, both of the 

 segments of which are produced into strong spines. The re- 



