164 THE HARMONIES OF NATURE. 



cordance with their wants, are, with rare exceptions, longer, more 

 muscular, and stronger than in the actively-swimming decapods, 

 where the two elongated tentacles or peduncles are the chief 

 organs of prehension. In some species we find the arms distinct 

 in others they are united by a membrane. Some have a 

 double row of suckers on each arm, others four rows, others 

 again but one. 



So wonderful are the variations which Nature, that consummate 

 artist, plays upon a single theme so inexhaustible are the modi- 

 fications she introduces into the formation of numerous species, 

 all constructed upon the same fundamental plan, and all equally 

 perfect in their kind ! Besides their arms, by help of which the 

 cephalopods either swim or creep, the forcible expulsion of the 

 water through the air-tube serves them as a means of locomotion 

 in a backward direction. By those which have an elongated- 

 body and comparatively strong muscles, this movement is per- 

 formed with such violence that they shoot like arrows through 

 the water, or, even like the flying-fish, perform a long curve 

 through the air, so as not seldom to bolt right over a sailing 

 ship, like a sportsman over a five-barred gate. Finally, the fin- 

 like expansion of their mantle, which in some species runs along 

 the sides of the body, and in others forms a kind of terminal 

 paddle, renders the nimble decapods good service in propelling 

 them forwards in the water. 



When a cephalopod has got hold of a fish or crab, the arms, 

 by sucking or hooking, instantly convey the luckless animal to 

 the mouth, where it is pitilessly crushed by two powerful 

 horny or calcareous jaws, fitting one over the other like the 

 mandibles of a tortoise. It might be supposed that the cepha- 

 lopods, by their swiftness, their arms, and their powerful jaws, 

 were sufficiently provided with the means of attack or defence ; 

 but Nature has besides favoured most of them with a remarkable 

 secretory organ, producing a black fluid and opening into the 

 air-tube. When pursued by its enemies, the animal ejects a 

 sufficient quantity of this inky liquid to form a thick cloud 

 in the water, which serves to conceal it from its foe. 



The enormous numbers and prodigious variety of the mol- 

 luscs are sufficient proofs of their importance in the economy 

 of Nature. The terrestrial snails are disseminated in more than 

 2,200 species over the surface of the globe; the slimy slugs infest 



