370 The Pedigree of Insects 



be explained as the result of convergence, and the 

 distinction between aquatic (branchiate) and terrestrial 

 (tracheate) arthropods breaks down when we remember 

 that animals with either kind of habit must be included 

 in the single class of the Arachnids. It is desirable 

 therefore to look for points of affinity beween Insects 

 and Crustaceans. The Crustacea are a large and varied 

 class, containing many orders, which are grouped into 

 two great series. The Entomosti-aca include Crustacea 

 with a very divergent number of segments and append- 

 ages, sometimes as many as thirty, sometimes as few 



.--• as seven pairs be- 

 ing present. The 

 higher orders of 

 Crustacea, on the 

 other hand, are 

 grouped together 

 as the Malacos- 

 traca which, with- 

 out exception, 

 appear to be com- 

 posed of twenty ^ 

 primitive seg- 

 ments, whereof 



Fig. 183. — Isopod Ciustacea.n, Li^iaoceanzca. From nineteen Carry 



Scharff, Irish Nat., vol. 2. paired limbs. A 



head region with two pairs of feelers, a pair of mandi- 

 bles, and two pairs of maxillce, is very general among 

 the Crustacea. The reader will hardly need to be re- 

 minded that the ancestors of the Insects and Arachnids 

 in all probability were animals with twenty segments 

 and nineteen pairs of limbs, whereof the first two pairs 

 were feelers ; of these the Insects, Centipedes and Mil- 

 lipedes, have lost the second pair, the Arachnids both 



^In all cases the number of segments stated does not include the pro- 

 cephalic lobes, or pre-oral segment (in front of the primitive mouth). 



