GENERAL INTRODUCTION 21 



sharply divided from the arms which always number five, 

 though they may divide and subdivide considerably, and 

 without the groove which always runs along the underside 

 of the arms in the starfish ; the sea urchins (Echinoidea) 

 which are all globular, heart-shaped or disc-shaped, with 

 a firm skeleton or shell covered with spines and usually 

 with definite rows of tube-feet passing through , the sea 

 cucumbers, sea gherkins or trepang (Holothuroidea) with 

 elongated, sausage-shaped bodies traversed generally by 

 five rows of tube-feet ; and finally the feather-stars and 

 sea lilies (Crinoidea) usually attached and with a central 

 disc with attachments below, and above a series of five, 

 often branching, arms which wave about in the water. 



The Arthropoda include the largest number of species 

 of any group in the animal kingdom. Like the Annelid 

 worms they have segmented bodies, but attached to all, 

 or many, of the segments are the jointed limbs which give 

 the group their name. Of the four great subdivisions, 

 three — the Insect, Spider and Centipede families — are 

 almost as exclusively composed of land animals, as the 

 fourth, called the Crustacea, are marine, one of the few 

 examples of the latter found on land being the common 

 woodlouse. There is no space to go into the many sub- 

 divisions of the Crustacea, so all-embracing a group that it 

 includes the tiny water fleas, the minute Copepods which 

 drift about in countless millions in the surface waters, the 

 barnacles which cover rocks and the bottoms of ships, the 

 sand-hoppers of the shore, the ghost-shrimps, the true 

 shrimps and prawns, the lobster and crayfish, the hermit 

 crabs and the many kinds of true crabs. The greatest 

 difference between the simpler and more complex types is 

 that in the latter the limbs, instead of being very much 

 alike from one end of the body to the other, have become 

 specialized into a number of feeding limbs near the mouth, 



