214 MANUAL OF ZOOLOGY. 



Decapoda, of which the latter includes all the highest and most 

 familiar examples of the class Crustacea. 



ORDER I. STOMAPODA. In this order there are generally 

 from six to eight pairs of legs, and the branchiae, when present, 

 are not enclosed in a cavity beneath the thorax, but are either 

 suspended beneath the abdomen, or, more rarely, are attached 

 to the thoracic legs. The shell, also, is thin, and often mem- 

 branous. From all the preceding orders the Stomapoda are, of 

 course, distinguished by the possession of pedunculate eyes. 



All the Stomapods are marine, and Squilla mantis may be 

 taken as a good example of the order. In this Crustacean the 

 carapace is small, and the posterior half of the thorax is un- 

 protected. Several of the anterior appendages are developed 

 into powerfully prehensile and hooked feet. The branchiae are 

 attached to the first five pairs of abdominal feet. The three 

 posterior thoracic and the abdominal appendages are in the form 

 of " swimmerets," and the tail is expanded into a powerful fin. 



ORDER II. DECAPODA. The members of this order are the 

 most highly organised of all the Crustacea, as well as being 

 those which are most familiarly known, the Lobsters, Crabs, 

 Shrimps, &c., being comprised under this head. For the most 

 part they are aquatic in their habits, and they are usually pro- 

 tected by strong,, resisting shells. There is always a complicated 

 set of " gnathites/' or appendages modified for masticatory pur- 

 poses, surrounding the mouth. The ambulatory feet are made 

 up of five pairs of legs (hence the name of the order), the first 

 pair and often some other pairs behind this being " chelate," 

 or having their extremities developed into nipping claws. The 

 branchiae are pyramidal, and are contained in cavities at the 

 sides of the thorax. The carapace is large, covering the head 

 and thorax, and the anterior part of the abdomen. 



The Decapoda are divided into three tribes, tenned respec- 

 tively the Macrura, Anomura and Brachyura, and characterised 

 by the nature of the abdomen. 



TRIBE A. MACRURA. The " long-tailed " Decapods included 

 in this tribe are distinguished by the possession of a well* 

 developed abdomen, often longer than the cephalothorax, the 

 posterior extremity of which forms a powerful natatory organ 

 or caudal fin. This section comprises the Lobster, Cray-fish, 

 Shrimp, Prawn, &c., of which the Lobster may be taken as 

 the type. 



In the Lobster (fig. 71) the somites of the head and thorax 

 are amalgamated into a single mass, the "cephalothorax," 

 covered by a carapace or shield, which is developed from " the 

 lateral or epimeral elements of the fourth cephalic ring, which 



