258 THE STUDY OF ANIMAL LIFE CHAP. 



rebuilt, wings bud out, the appendages of the adult are 

 formed, and out of the pupal husk there emerges an 

 imago, an insect fully formed. 



Arachnid a. Spiders, Scorpions, Mites, etc. This class 

 is unsatisfactorily large and heterogeneous. In many 

 the body shows two regions, (1) the fused head and breast 

 (cephalothorax), with two pairs of mouth parts and four 

 pairs of walking legs, and (2) the abdomen with no ap- 

 pendages. Respiration may be effected by the skin in 

 some mites, by tracheae in other mites, by tracheae plus 

 " lung-books " in many spiders, by ' lung-books ' alone 

 in other spiders, by " gill-books " in the divergent king- 

 crab (Limulus). 



The scorpions with a poisoning weapon at the tip of 

 the tail (see fig. 7), the little book-scorpions (Chelifer), 

 the long-legged harvest - men (e.g. Phalangium) ; the 

 spiders proper spinners, nest-makers, hunters ; the 

 mites, and many other types are ranked as Arachnids. 

 The quaint king-crab (Limulus) last of a lost race has 

 some affinities with scorpions and with the extinct Euryp- 

 terids. The ancient Trilobites, which lasted from the 

 Cambrian to the Carboniferous, exhibit some affinities 

 with the king-crab, but the nature of the limbs and the 

 presence of antennae suggest relationship with Crustaceans. 



7. Molluscs. It seems strange that animals, the 

 majority of which are provided with hard shells of lime, 

 should be called mollusca ; for that term first used by 

 Linnaeus is a Latinised version of the Greek malakia, 

 which means soft. Aristotle applied it originally to the 

 cuttlefish, which are practically without shells, so that 

 its first use was natural enough, but the subsequent 

 history of the word has been strange. 



Cockle, mussel, clam, and oyster ; snail and slug, 

 whelk and limpet ; octopus, squid, and pearly nautilus ; 

 what common characteristics have they ? Most of 

 them have a bias towards sluggishness, and on the 

 shields of lime which most of them bear, do we not 

 read the legend, "castles of indolence"? But this 

 sluggishness is only an average character, and the shell 

 often thins away. The scallop (Pecten) and the swimming 



