xxvii GENERAL STRUCTURE 309 



spiders, and mites ; centipedes and millipedes ; and all 

 kinds of insects, such as cockroaches, beetles, flies, ants, 

 bees, butterflies, and moths. A crayfish forms a very fair 

 type of the group. 



In the phylum Mollusca are included the ordinary bi- 

 valves, such as mussels and oysters ; snails, slugs, and other 

 univalves or one-shelled forms ; sea-butterflies ; and cuttle- 

 fish, 1 squids, and Octopi. An account of a fresh-water 

 mussel will serve to give a general notion of the character 

 of this group. 



Finally, under the head of Vertebrata are included all the 

 backboned animals : the lampreys and hags ; true fishes, 

 such as the shark, skate, sturgeon, cod, perch, trout, c. : 

 amphibians, such as frogs, toads, newts, and salamanders ; 

 true reptiles, such as lizards, crocodiles, snakes, and tor- 

 toises ; birds ; and mammals, or creatures with a hairy skin 

 which suckle their young, such as the ordinary hairy 

 quadrupeds, whales and porpoises, apes, and man. The 

 essential structure of a vertebrate animal will be understood 

 from a brief description of a dog-fish. 



THE STARFISH. 1 



A common starfish consists of a central disc-like portion, 

 from which radiate five arms or rays. It crawls over the 

 rocks with its ventral surface downwards, its dorsal surface 

 upwards. It can move in any direction, so that, in the 

 ordinary sense of the words, anterior and posterior extremi- 

 ties cannot be distinguished. Radial symmetry such as this, 

 i.e., the division of the body into similar parts radiating from 

 a common centre, is characteristic of the Echinodermata 

 generally. 



1 For a detailed description of a Starfish, see Rolleston and Hatchett 

 Jackson, Forms of Animal Life (Oxford, 1888), pp. 190 and 311. 



