3o6 THE STARFISH less. 



The phylum Arthropoda includes crayfishes, lobsters, 

 crabs, shrimps, prawns, wood-lice, and water-fleas ; scorpions, 

 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 

 example of the aquatic kinds {Crustacea). 



In the phylum Mollusca are included the ordinary bi- 

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

 univalves or one-shelled forms ; and cuttle-fishes, 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. 



The commonest British starfish is Astcrias rubens, but 

 the main features of the following description will apply to 

 any species. The starfish consists of a central disc-like 

 portion, from which radiate five arms or rays. The animal 

 crawls over the rocks with its flat, light-coloured ventral 

 surface downwards, and with its darker, convex, dorsal 

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

 the ordinary sense of the words, anterior and posterior ex- 

 tremities cannot be distinguished. Radial symmetry such 



