156 INTRODUCTION TO ZOOLOGY chap. 



increase in activity noticeable in them in warm weather, and 

 when the sun shines on them. 



. The means by which the blood is aerated 



■ varies according to the habitat of the different 

 Arthropods. Crustaceans (crabs, lobsters, etc.) live always in 

 the water, and therefore need the respiratory organs termed 

 gills, which are adapted for the purpose of obtaining oxygen 

 from that dissolved in the water. Arachnids (spiders) and 

 Insects, on the other hand, either live on land, or, if in water, 

 come to the surface to breathe, and they all have special air- 

 tubes or tracheae which carry the air to all parts of the body 

 (see p. 210). 



Most Arthropods undergo a more or less 

 ment^ extensive metamorphosis, i.e. from the egg there 

 hatches a little form very unlike the adult parent 

 of the egg, and this young form or larva only gradually 

 develops into the adult form, the metamorphosis being accom- 

 panied by frequent moultings of the inelastic chitinous skin. 



This group of Arthropods is on the whole so 



Art^roBods ^^'^ sharply marked off from any of the other 



groups of animals, that it was long before a link 



connecting it with any other was recognised. This link is 



Fig. 94. — Peripatus capensis. (After Balfour. ) 



now found in the little creature Peripatus, of which several 

 species are known in various tropical and southern countries, 

 and which combines in a curious way both Annelid and 

 Arthropod characteristics. It has an elongated vermiform 

 body which is not apparently segmented either externally or 

 internally. It beat's antennae on its head. There are two 

 jaws round its mouth, and a pair of little papillae on the 

 sides of the head, from which it can eject slime. The eyes 

 are simple, like those of worms rather than of Arthropods. 

 Behind the head there project, from the lower side of the 

 body, a number of paired appendages, on the end of each of 

 which is a pair of small claws. These legs, at first sight, 



