CHAPTER XII 

 THE RADIATE INDIVIDUAL 



This Individual type comprises a group of marine organisms 

 biologically termed the Echinodermata, of which familiar examples 

 are the Starfishes and the Sea-urchins ; and although they do not 

 exemplify a further advance on the main road of Continuity's evolu- 

 tion, yet they do represent a higher form of Continuity than mega- 

 zooidal and prepare us in a sense for the Segmental Individuals of 

 the main road. In fact, it is possible to regard the Radiate as humble, 

 or it may be depraved, Segmental Individuals for several reasons. 

 Our brief remarks will deal with the common Starfish. 



For a detailed account of this animal's structure the reader must 

 refer to a textbook of Zoology, for we are only dealing with out- 

 standing features. 



The organism's shape is that of a five-pointed star ; the 

 arms, comparatively short and thick, radiating outwards 

 with their tapered points equidistant from their neighbours. 

 The animal does not swim, but moves about at the bottom 

 of the water by means of many protrusible tube-feet on 

 the under surface of the arms. The mouth is in the centre 

 of the under or ventral surface of the body, while on the 

 upper or dorsal the anus opens. The digestive tract consists 

 of a mouth, gullet, a pouched stomach, and beyond this 

 a " pyloric sac," from each of whose five corners a duct 

 arises to enter one of the arms and bifurcate into a pair 

 of blind pouches or " pyloric caeca." Beyond the pyloric 

 sac a straight intestine leads to the dorsally situated anus. 

 More or less completely surrounding the alimentary tract 

 is a true ccelomic cavity. 



The Starfish has a water-vascular system, or, as it might 

 be called, a radial canal system of a higher type than that 

 possessed by the Medusa. It is concerned with the function 

 of locomotion and does not communicate with the digestive 

 tube. There is a circular canal round the gullet, and from 

 this radiate five straight canals down the floor of the arms. 



107 



