omous, the metameres being differently modified in different parts 

 of the body, and there is more or less extensive fusion of metameres, 

 giving rise to patterns sometimes characteristic of large divisions. 

 The appendages when present are also modified into organs of a 

 great variety of functions, and are distinguished by their jointed 

 character, associated with the development of a hard external or 

 exo-skeleton. 



The common crayfish, Cambarus (Eur. Astacus), may be taken 

 as an example of the lower Arthropoda, the Crustacea, the mem- 

 bers of which are for the most part aquatic, breathing by means 

 of gills. In the crayfish and numerous closely allied crustaceans 

 both the segmentation and limb modification are highly organized, 

 but there are numerous forms less completely organized, more 

 generally or partly segmented, some of which make up a large 

 portion of the microscopic life or plankton of the water, while 

 others are common parasites of the external surface and gills of 

 fishes (parasitic Copepoda). 



The Myriapoda or centipedes are terrestrial arthropods in 

 which gills are replaced by air-tubes analogous to those of insects, 

 but in which the segmentation and appendages are remarkably 

 uniform as in annulates. A few have been known to live in the 

 nasal cavity and intestine of man. Many are capable of inflicting 

 painful and sometimes dangerous bites through the modification 

 of an anterior pair of appendages into curved perforated poison 

 claws, which carry the secretion of poison glands into the wound 

 inflicted. 



Arachnida, mites, ticks, spiders, and scorpions are air- 

 breathing, for the most part terrestrial arthropods, provided, except 

 in most of the mites, with air-tubes or with respiratory plates or 

 book-lungs. The body is typically divided into a small anterior 

 cephalothorax and a larger posterior abdomen. The cephalo- 

 thorax contains six fused segments bearing characteristic appen- 

 dages. The first appendages, chelicerae, are curved poison claws, 

 the second, pedipalpi, are jointed appendages in relation to the 

 mouth, the remaining four pairs are walking legs. The higher 

 arachnids or spiders proper normally use their chelicerae for killing 

 their insect prey but they are frequently able to attack larger 

 animals and to inflict poisonous bites. 



The Acarina are smaller arachnids, commonly known as mites 

 and ticks, and significant both as parasites and as the carriers of 

 protozoan parasites. The external division of the body into 

 cephalothorax and abdomen is not in evidence, but the character- 

 istic appendages are present though in a modified form. Thus the 

 chelicerae and pedipalpi are modified for piercing and sucking, 



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