ANNTJLOSA: CRUSTACEA. 191 



found in the sea (Sphceroma). In the Sedentary Isopoda the 

 animals are all parasitic, with short, incurved, hooked feet. 

 This section includes the single family of the Bopyridce, all the 

 species of which live parasitically either in the gill-chambers, 

 or attached to the ventral surface, of certain of the Decapod 

 Crustacea, such as the Shrimps (Grangones) and the Palcemones. 



The Cursorial, or running, Isopods mostly live upon the land, 

 and are, therefore, destitute of swimming feet. The most fa- 

 miliar examples of this section are the common Wood-lice 

 (Oniscus). Here, also, belongs the little Limnoria terebrans, 

 so well known for the destruction which it produces by boring 

 into the wood-work of piers and other structures placed in 

 the sea. 



DIVISION B. PODOPHTHALMATA. The members of this division 

 have compound eyes supported upon movable stalks or pedun- 

 cles, and the body is always protected by a cephalothoracic 

 carapace. It comprises the two orders Stomapoda and Deca- 

 poda, 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 branchiee, 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 branchiaB 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 povrerful 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 protected 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 



