CHAPTER XIV 



THE INVERTEBRATES (continued) : ARTHROPODS 



AND MOLLUSCS 



The jointed-legged animals. None of the invertebrates 

 we have studied so far has any organs which are really of 

 the nature of legs, in the meaning commonly ascribed to the 

 word. In fact the animals of only one out of the eleven 

 invertebrate branches have organs which show any analogy 

 with the jointed legs of our own body and of the other ter- 

 restrial vertebrates. This branch is that of the Arthropoda 

 which includes in its five classes a great host of familiar 



" v T <*V^ " - ^ 

 FIG. 61 Per I pains eiseni. Mexico.) 



animals, namely, all the crabs, shrimps, sand-fleas, barnacles, 

 etc., (class Crustacea)', the thousand-legged worms (bad 

 name; they are not worms at all) and centipeds (class 

 Myriapoda) ; all the insects (class Insecta) ; the scorpions, 

 spiders, mites and ticks (class Arachnida) ] and one other 

 class, the Onychophora, containing but a single genus 

 (Peripatus) of curious half worm-like, half centipede-like 

 animals found in the tropics which seem to be a sort of con- 

 necting link between the true worms and the myriapods. 

 All of these animals agree in possessing legs composed of 

 several distinct articulated segments. 



The crayfish, or crawfish (whose structure is described 

 in Chapter IV) is a good example of the Crustaceans. It 



149 



