ARTHROPODS. i^o 



ARTHROPODS. 



In number of species, the present group far outranks all the other 

 divisions of the animal kingdom. Statistics are proverbial]}- dry read- 

 ing, but a few figures will show how large the present group is. The 

 molluscs, which we have just left, are represented by about twenty 

 thousand species in the whole world ; there are about twelve thousand 

 different kinds of birds, and ten thousand fishes ; but these numbers 

 are comparatively insignificant when we take the arthropods into con- 

 sideration. Of beetles alone, one catalogue enumerates seventy-seven 

 thousand and eight species ; the bugs are estimated at fifty thousand, 

 and the butterflies and moths at half as many more. In short, the 

 total number of arthropods may be safely set at between a quarter and 

 half a million species, and probably nearer the larger than the smaller 

 of these limits. 



The arthropods betray their vermian ancestry to a greater extent than 

 do the molluscs or the vertebrates. They have the same ringed body, 

 and essentially the same internal structure as do the worms, but they have 

 certain other features which mark them off strongly from their relatives. 

 From a superficial point of view, the most prominent of these differences 

 is that implied in the name of the group. Arthropoda is but the Greek for 

 jointed feet, and all the members of the group, except those excessively 

 degraded by parasitic habits, show, to even the most casual observer, this 

 structure of the appendages. But if we study the development of the 

 parasitic forms, even these are found to be no exceptions to the rule, for 

 in their young stages their feet are jointed just like those of any other 

 form. They have the same hardened skin, which is interrupted at points 

 to allow one portion to move on the other ; but with growth and the 

 adoption of parasitic habits these are lost, to a greater or less extent, until, 

 finally, in some forms the adult does not retain even the slightest char- 

 acter that would suggest relationship to the typical arthropods. 



Concerning the arthropods there are many interesting and important 

 points, but we must forego mention of all but two in this connection, — 

 the compound eyes, and the wonderful metamorphoses through which many 

 of these animals pass in acquiring their adult condition. There are several 



