428 ECHINODERMATA 



and the higher Invertebrata. Like so many of the latter, the 

 Echinodennata have an anus, that is, a second opening to tlie 

 alimentary canal through which indigestible material is rejected ; 

 like them also, they have a body-cavity or coelom surrounding 

 the alimentary canal — from the lining of which the genital cells 

 are developed. On the other hand, there is no definite circulatory 

 system, nor any specialised excretory organ, and the nervous 

 system exhibits no concentration which could be called a brain, 

 and is, moreover, in close connexion with the skin. In all 

 these points the Echinodermata resemble the Coelenterata. 



One of the most characteristic features of the internal 

 anatomy of Ecliinodermata is the presence of a peculiar series 

 of organs, known collectively as tlie water-vascular system or 

 hydrocoel. This is really a special division of the coelom 

 or liody -cavity which takes on the form of a ring-shaped canal 

 embracing the mouth, from which are given off long radial 

 canals, usually five in number, running to the more peripheral 

 parts of the body.^ Each radial canal carries a double series of 

 lateral branches, which push out the skin so as to appear as 

 appendages of the body. These appendages are known as 

 tentacles or tube-feet ; they are both sensory and respiratory 

 in function, and often in addition, as the name tube-foot 

 indicates, assist in locomotion. As a general term for these 

 appendages, to be applied in all cases without reference to their 

 function, the name podium has l)een suggested and will be 

 employed here. A system of canals, in many ways resembling 

 the water-vascular system, is found in Brachiopoda, Gephyrea 

 and Polyzoa, but the peculiarity of Echinodermata is the way 

 in which it is ke})t filled with fluid. From the ring-canal in the 

 interval (or interradius) between two radial canals, a vertical 

 canal, termed the stone-canal, is given off, which communicates 

 with the exterior l)y means of a sieve-like plate, the madre- 

 porite, pierced l)y fine canals. These canals and the stone-canal 

 itself are lined with powerful cilia, which produce a strong inward 

 current, and keep the water- vascular system tensely filled with 

 sea water. 



The phylum includes the familiar Starfish and Sea-urchins, 

 which in sheltered spots are found between tide-marks ; the 



^ In the Synaptidae the radial canals although present in the young are lost 

 in the adult (Ludwig, 1892, in Bronn's Tkier-Iieich, Bd. ii. Abt. 3, Bach i. p. 460). 



