INSECTS 343 



B. — Aquatic Forms (Branchiata). 



Class 5. Crustaceans (CRUSTACEA). — Lobsters, Crabs, Shrimps, &c. 

 Class 6. King-Crabs (Xiphosura). 

 Class 7. Sea-Spiders (PycnogoNIDA). 



A.— AIR-BREATHING ARTHROPODS (Tracheata) 

 Class i.— INSECTS (Insecta) 



This class embraces an astonishing number of species, more 

 numerous, in fact, than those of all other groups of land animals 

 put together. Yet, in spite of this, they do not exhibit so wide a 

 range of characters as might be expected, so that the study of a 

 carefully-chosen type forms an intelligible key to the entire class. 

 The too -familiar Cockroach or Black "Beetle" [Periplaneta 

 orientalis) furnishes just such a type, though the American 

 species {P. Americana) obtainable in many seaports, is decidedly 

 better on account of its larger size. 



External Characters (fig. 201). — The body is obviously 

 divided into three regions — head, thorax, and abdomen — the dis- 

 tinction between the first two being emphasized by the presence 

 of a narrow neck. In such an insect as a wasp the demarcation 

 between thorax and abdomen is equally sharp, while in many 

 insects, on the other hand, the three regions are bounded by a 

 continuous curved outline unbroken by constrictions. 



Each one of these three regions of the body is made up of rings 

 or segments, which differ very much among themselves in char- 

 acter, and in some places are so closely fused together that their 

 exact number cannot be definitely made out. Arthropods, in 

 fact, or any large group of them, furnish innumerable instances of 

 the phenomena described elsewhere (see p. 195) in reference to 

 the skeleton of the limbs, where all sorts of modification of a 

 common plan may arise by unequal development of parts, fusion, 

 and reduction. One of the lower Arthropods, for example, such 

 as a Centipede or Peripatus, consists of a large number of similar 

 segments which have only undergone great specialization at the 

 head end, while in such highly specialized forms as Insects the 

 segments have been much reduced in number, and are grouped 

 into regions which, in correspondence with special uses, have 

 acquired special characters. 



