ANNULOSA : CRUSTACEA. 227 



Division B. Podophthalmata. — The members of this di- 

 vision have compound eyes supported upon movable stalks or 

 peduncles, and the body is always protected by a cephalotho- 

 racic ca.rapace. Most of the Podophthalma pass through Zoea- 

 stages in their development. It comprises the two orders 

 Stomapoda and 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. 

 The development of the Stomapoda would appear to be by 

 means of " Zoeae." 



All the Stomapods are marine, and the Locust Shrimp 

 (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 unprotected. Several of the anterior ap- 

 pendages are developed into powerfully prehensile and hooked 

 feet. The branchia are attached to the first five pairs of ab- 

 dominal feet. ' The three posterior thoracic and the abdominal 

 appendages are in the form of " swimmerets," and the tail is 

 expanded into a powerful fin. Besides the Locust Shrimps, 

 the order includes the Glass Shrimps {Erichthys) and their 

 allies, and the Opossum Shrimps (Afysis). 



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 com- 

 plicated set of " gnathites," or appendages modified for mas- 

 ticatory purposes, 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 con- 

 tained 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 heart of the Decapoda is in the form of a 

 more or less quadrate sac, furnished with three pairs of valvular 

 openings. As regards the development of the Decapods enor- 

 mous differences obtain, even amongst forms very closely 

 allied to one another. 



