60 STARFISHES 



of remarkable changes develops into the radially symmetric 

 adult. 



The most characteristic feature of the internal anatomy of 

 the Echinoderma is the presence of a series of organs which 

 together form what is called the " water- vascular system " (hydro- 

 ccel). This is a special division of the body-cavity or ccelom, 

 in the form of a ring-shaped canal surrounding the mouth, and 

 giving off long, radial canals, usually five in number, which run 

 to the outer parts of the body. Each of these radial canals carries 

 a double series of lateral branches, which push out the skin so as 

 to appear as appendages of the body and are the tube-feet by 

 means of which the animal moves along the floor of the sea; 

 they also function as sensory and respiratory organs. From the 

 ring-canal in the space between two of the radial canals a vertical 

 one, called the stone-canal, is given off, which communicates 

 with the exterior by means of a porous plate pierced by fine 

 canals, which from a fancied resemblance of its outer surface, 

 covered with wavy lines, to a coral, is called the madreporite or 

 madreporic body. Powerful cilia, capable of producing a 

 strong inward current, line these canals and the stone-canal, so 

 that the whole water-vascular system is kept tensely filled with 

 sea-water. 



The Echinoderma are divided into two great sub-phyla or 

 subdivisions : (i) the Pelmatozoa, 1 or Echinoderms provided 

 with a stalk by which the animal is attached to a base, either 

 throughout its life, or in the larvas stages dnly ; and (2) the 

 Echinozoa, 2 or Echinoderms without stalks at any time of their 

 life. To the first belong the beautiful living Feather Stars (Cri- 

 noided), and the extinct forms Blctstoidea and Cystoidea ; while 

 to the latter belong the Common Starfish, the Brittle and Sand 

 Stars, the Sea Urchins, and the Holothurians. 



We may take as our first example of the Echinoderms, and 

 of the first class, the Asteroidea, 3 of the Echinozoa, the common 

 reddish-coloured Five -fingered Starfish (Asterias rubens), to be 



1 Greek, pelma, a stalk ; zoon, animal. 



2 Greek, echinos, hedgehog ; zoon, animal. I have retained the name Echinozoa 

 for this sub-phyla, as it is still used in the British Museum (Natural History) Guide 

 to designate it, and as it is more familiar and descriptive than the term Eleutherozoa, 

 now adopted by many authors. 



3 Greek, aster, a star ; eidos, form. 



