THE OTHER BEASTS 337 



or '' back-boned " animals, there are various other such 

 primary divisions, as for example the sub-kingdom of 

 molluscs or mollusca (squids, snails, oysters, (fee); the 

 sub-kingdom of creatures with jointed legs or arthropoda 

 (crabs, shrimps, insects, spiders, scorpions, hundred-legs, 

 &c.); the sub-kingdom of worms or vermes; that of star- 

 fishes or echinoderma, and so on. But with all these we 

 have here nothing further to do ; we have but to recog- 

 nise the fact that our own sub-kingdom is but one of a 

 certain number of other such primary groups. 



In our essay on the opossum,* the fact was roughly 

 indicated that the division of " back-boned animals" was 

 made up of certain classes — namely, mammals, birds, 

 reptiles, and fishes ; but in that on the bull-frog, it was 

 further pointed out that frogs, efts, and their allies, 

 should more properly constitute a group by themselves, 

 and so constitute a distinct class, "batrachia." 



Now these five classes may be taken as falling into 

 two sections which are very distinct, but w^hich it will 

 here sufiice to characterise (apart from exceptional leg- 

 less forms) as, (i) those having limbs made up of arm 

 and hand, or leg and foot respectively, and (2) those the 

 limbs of which are not so made up. The limbs of all 

 beasts, birds, reptiles, and batrachians, are thus com- 

 posed, but those of all other back-boned animals are not 

 thus composed ; therefore, they arc not so composed in 

 fishes, and so fishes are not included amongst the animals 

 noticed in these essays. 



The groups of the class of birds, have been more or 

 less indicated in our study of the turkey,! those of the 

 class of reptiles have been considered with the rattle- 

 snake,! and those of the class batrachia in our notice 

 of the bull-frog. § 



" See p. 42. t See p. 66. % See p. 122, § See p. 96. 



Y 



