xi PHYLUM ARTHROPODA 555 



Sub-order 1 . Macrura. 



Decapoda with well-developed, elongated abdomen, which is 

 usually held in the extended position, and terminates in an expanded 

 fan-like tail-fin composed of the telson and the uropods. The 

 eyes are not enclosed in orbits. The antennules and antennae 

 are both large ; the former are not sunk in pits, and the antennae 

 usually have a scale-like exopodite (squame). 



Included among the Macrura are (a) swimming forms Penceus 

 and Palcemon (Prawns), Crangon (Shrimps), Lucifer, etc. ; and 

 (b) creeping forms Homarus (Lobster), Astacus, Astacopsis, Para- 

 nephrops, Cambarus (Fresh-water Crayfishes), Palinurus (Rock- 

 lobsters), Soyllarus, etc. (Figs. 472, 473). 



Sub-order 2. Anomura. 



Decapoda with the abdomen more or less reduced, usually held 

 in a flexed position, and not provided with such a well-developed 

 tail-fin as in the Macrura. 



In most respects the Anomura are intermediate between the 

 Macrura and the Brachyura. Examples are the Hermit-crabs 

 Pagurus (Fig. 474) and other genera, the Cocoa-nut crab Birgus, 

 Galathea, Hippa, Porcellana, etc. 



Sub-order 3. Brachyura. 



Decapoda in which the abdomen is greatly reduced, shorter 

 than the cephalothorax, and permanently flexed beneath it. The 

 antennules and the eyes are both capable of being retracted into 

 cavities. There is a metamorphosis comprising zocea and megalopa 

 stages. 



Including the true Crabs such as Cancer, Maia, Grapsus, etc. 

 (Figs. 475, 476). 



Division 4. Hoplocarida. 



Malacostraca in which the carapace does not coalesce with 

 at least the last four thoracic segments, so that the cephalothorax 

 is relatively short. In front of the head proper are two movable 

 segments, one bearing the stalked eyes, the other the antennules. 

 The branchiae are borne on the abdominal appendages. The heart 

 is elongated. There is a metamorphosis, but a nauplius stage 

 is not known to occur. 



ORDER STOMATOPODA. 



This, the only order of Hoplocarida, includes Squilla (Fig. 477), 

 Gonodactylus , and other genera. 



Sytematic Position of the Examples. 



The genera Apus and Lepidurus belong to the family Apodidce 

 of the order Notostraca of the sub-class Branchiopoda. 



