CHAP. XV Backbo7ieless Anivials 243 



out, the appendages of the adult are formed, and out of the pupal 

 husk there emerges an imago, an insect fully formed. 



{i) Araclmida. — Spiders, Scorpions, Mites, etc.^ — This class is 

 unsatisfactorily large and heterogeneous. In many the body is 

 divided into two regions, the head and breast (cephalotliorax), with 

 two pairs of mouth parts and four pairs of walking legs, the 

 abdomen with no appendages. Respiration may be effected by 

 the skin in some mites, by trachea 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. 



The scorpions with a poisoning weapon at the tip of the tail, 

 the little book-scorpions {Chelifer), the long-legged harvest-men 

 (e.g. Phalangiuiii); the spiders proper — spinners, nest -makers, 

 hunters ; the mites ; the strange parasite {Pentasiointnn) in the 

 dog's nose ; the quaint king-crab [Linitelus) — last of a lost race, 

 with which the ancient Trilobites and Eurypterids were connected ; 

 all these are usually ranked as Arachnids ! 



6. 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 lim- 

 pet ; octopus, squid, and pearly nautilus ; what common character- 

 istics 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 Lima are active compared with the 

 oyster, and they have thinner shells ; the snails which creep slowly 

 between tides or on the floor of the sea are heavily weighted, while 

 the sea-butterflies (Pteropods) have light shells, and most cuttlefish 

 have none at all. 



The shell is very distinctive, but we are not able to state 

 definitely how it is formed or what it means. In most of the 

 embryo molluscs which have been studied there is a little pit or 

 "shell-gland" in which a shell begins to be formed, but the shell 

 of the adult is in all cases made by a single or double fold of skin 

 known as the " mantle." In some cases where the shell seems to 

 be absent, e.g. in some slugs, a degenerate remnant is still to be found 

 beneath the skin, while in other cases [e.g. most cuttlefish) its 

 absence is to be explained as a loss, since related ancestral species 

 possess it. There are, however, two or three primitive forms 



