534 CRUSTACEA ORDERS ARTHROSTRACA AND THORACOSTRACA. 



pendages, composing the head and thorax ; 6 abdominal segments, also 

 frequently bearing jointed appendages ; and a tail-piece. They have two 

 antennae, and two or four eyes. They differ very much in shape and 

 appearance, but include nearly all the Cn^tacea which fall under ordinary 

 observation. 



ORDER I. ARTHROSTRACA. 



The ArtJirostraca are small or moderate-sized Crustacea, with two compound 

 eyes, which are not stalked. There are two pairs of antennae, three pairs of 

 jaws, and one pair of foot-jaws attached to the head. The thorax usually 

 consists of seven segments (rarely less), each bearing a pair of legs. The 

 abdomen likewise generally consists of six segments bearing legs, and a 

 terminal segment, but is sometimes much modified and aborted. 



The Arthrostraca are divided into two sub-orders, and these again into 

 numerous families. 



Sub-Order I. Isopoda. 



In the Isopoda the head is distinctly separated from the thorax. The body 

 is broad, and somewhat arched, and there are usually 7 thoracic segments 

 bearing legs ; but the abdomen is short and often imperfectly developed, 



bearing lamellated gill-feet. In some fami- 

 lies, such as the Bopyridce, and Cymothoidce, 

 which are parasitic on other Crustacea, or oil 

 fishes, the legs are armed with claws. But 

 the most familiar family of the Isopoda is the 

 Oniscidce, which includes the creatures known 

 as "wood-lice," "sows" or "sow-bugs." They 

 are short, broad, brown scaly creatures, with 

 conspicuous antennae, and a body arched 

 above, and flattish below. Some are able 

 to roll themselves up into a ball. They are 

 about half an inch or less in length, and 

 are found in damp, dark places among moss, 

 under the loose bark of trees, or in cellars. 

 Onelarge species, overan inch in length, Lygia 

 oceanica (Linn.), is marine, and is met with 

 running over rocks between tide marks. As 

 I have already mentioned elsewhere, a speci- 

 men which was found running along a tow- 

 rope in Dublin Harbour during the Colorado 

 Beetle scare some years ago, was secured and 

 announced in the papers as the dreaded Colorado Beetle just landing from 

 America ! 



Sub-Order II. Amphipoda. 



In the Amphipoda, the head and the first thoracic segment are fused to- 

 gether, and the body is laterally compressed, and strongly arched. There are 



Fig. 8. LYGIA OCEANICA. 

 Nat. sue. 



