256 THE SEA SHORE 



CHAPTER XIII 

 MARINE ARTHROPODS 



THE sub-kingdom A rthropoda contains a vast assemblage of animals, 

 all of which, as the name implies, possess jointed appendages. 

 Their bodies are covered with a skin that is hardened by a horny 

 substance (chitiri), and frequently, also, by the deposit of carbonate 

 of lime. 



The body of Arthropods is made up of a chain of segments, all 

 of wh ch are built up on one common pattern, and each one is sur- 

 rounded by a ring of the hardened skin or exo-skeleton that gives 

 attachment to a pair of appendages. Commonly, however, two or 

 more of the segments become fused together, being covered by a 

 continuous plate or shield, in which the boundaries of the rings are 

 almost or completely obliterated ; but in such cases the appendages 

 they bear always remain distinct, so that the true number of seg- 

 ments is always apparent. The skin between those segments that 

 are not so fused together remains soft and flexible, thus allowing 

 the body to be freely bent. 



The appendages exhibit a great variety of structure, and are as 

 varied in their functions. Some are used as feelers, and others 

 as jaws for seizing or masticating food. Some are developed into 

 powerful seizing organs for purposes of defence or attack, some into 

 paddles for swimming, while others are legs adapted for walking. 



All these appendages are made up of segments, each of which, 

 like those of the body itself, is surrounded by a ring of hardened 

 skin, and connected with its neighbours by a flexible integument 

 that allows perfect freedom of movement ; while within are the 

 muscles, often very powerful, by which the appendage is moved. 



In the arthropods we have a sub-kingdom of highly organised 

 animals, with distinct, and often very complicated, systems of organs 

 for digestion, circulation, and respiration ; and the nervous system 

 consists of a well-developed chain of ganglia, connected by nerve 



