CYAMUS. ORDER ISOPODA. 261 



The Cyamus and its allies, belonging to the second division of 

 the Order, appear to be mostly parasitic in their habits. The 

 Cyamus, commonly termed the Whale-Louse, attaches itself, by 

 means of its strong claws, to the surface of the body of the 

 Whale; which is sometimes so completely covered by these 

 parasites, that the individuals thus infested may be easily 

 recognised at a considerable distance by their white colour. 

 When the parasites are removed, the skin of the Whale is found 

 to be deprived of its epidermis. 



ORDER V. ISOPODA. 



797. These Crustacea bear a general -resemblance to the 

 Amphipoda ; but their bodies are flattened horizontally, instead 

 of being compressed vertically, and the abdomen is not terminated 

 either by appendages adapted for leaping, nor by an expanded 

 fin for swimming, as in that group. The thorax nearly always 

 consists of seven segments, and is furnished with seven pairs of 

 appendages adapted for walking (Fig. 456) ; but the respiratory 

 organs are not attached to them, but are developed as expansions 

 of the extremities of the pro-legs, which are attached to the 

 abdominal segments. These organs, which have usually an oval 

 form and a membranous texture, are sometimes suspended freely 

 beneath the abdomen ; but they are occasionally covered in by 

 little scales, which fold over and protect them. The females 

 usually have large plates attached to the base of the thoracic 

 legs, which form, by their meeting, a pouch in which the eggs 

 and young are matured ; others have a membranous bag in this 

 situation. The newly-hatched young have only six thoracic 

 segments and six pairs of legs, acquiring an additional segment 

 and pair of legs at the time of their first moult. 



798. Of this Order, the principal part is aquatic ; but one 

 group is terrestrial. Many of the former are parasitic upon other 

 animals, very frequently upon larger Crustacea; this is the 

 case with the Bopyrus, which is parasitic upon the common 

 Prawn, affixing itself beneath the carapace, upon the branchiae, 



