142 ZOOLOGICAL PHILOSOPHY 



Here for the first time, animals from the outward aspect exhibit a 

 distinct head ; very remarkable, although still imperfect, eyes ; jointed 

 legs arranged in two rows ; and that symmetrical form of paired and 

 opposed parts that we shall henceforth find employed by nature up 

 to and including the most perfect animals. 



On examining the interior of insects, we also see a complete nervous 

 system, consisting of nerves which terminate in a ganghonic longitudinal 

 cord ; but although complete, this nervous system is still very imper- 

 fect, since the nucleus to which sensations are conveyed appears much 

 broken up, and the senses themselves are few and ill-developed ; lastly, 

 we see a true muscular system, and sexes which are distinct but which 

 as in plants can only provide for a single fertiUsation. 



It is true that we do not yet find any circulatory system ; and we 

 shall have to pass higher up the animal chain before we meet with 

 this improvement in organisation. 



It is characteristic of all insects to have wings in their perfect state ; 

 so that those which have none owe their condition to the fact that their 

 wings have become habitually and permanently aborted. 



Observations. 



In the table which I shall now give, the number of genera is greatly 

 reduced from what has been hitherto constituted among the animals 

 of this class. Such a reduction appears to me to be required in the 

 interests of study, and also of simplicity and clearness of method. 

 I have not carried it so far as to be detrimental to our knowledge 

 of the animals. If we were to utiUse every appreciable peculiarity 

 in the characters of animals and plants for indefinitely multiplying 

 their genera, we should, as I have already said, merely encumber and 

 darken science instead of serving it ; we should make the study of 

 it so comphcated and difiicult that it would only be practicable for 

 those who were ^villing to devote their entire life to gaining a knowledge 

 of the immense nomenclature and the minute characters selected for 

 marking the distinctions between these animals. 



TABLE OF INSECTS. 



(A) StroKiNG Insects. 



Their mouth has a sucking-organ with or without a sheath. 



Order 1. — Apterous Insects. 

 A bivalve, three-jointed, proboscis, enclosing a sucking-organ of two setae. 

 Wings generally abortive in both sexes ; larva without legs ; pupa motionless^ 

 in a cocoon. 



Flea 



