ECHINODERMATA 219 



is no brain. There is no heart or definite system of 

 blood vessels. There is, however, a remarkable water- Water- 

 vascular system, which consists of a series of tubes con- syst em 

 nected with tube feet or podia, especially conspicuous in 

 the starfish, where they serve for locomotion. In a 

 starfish or sea urchin a sievelike plate (madreporite) may 

 be found on the upper (aboral) surface. Through this 

 water passes into a canal, propelled by movements of 

 minute cilia. This canal or tube ends in a tubular ring, 

 from which proceed radially five tubes, following the 

 arms of the starfish, or ascending within the sides of the 

 sea urchin. Extending from these radial tubes are small, 

 hollow processes, the tube feet. The structure is some- 

 what more complicated than this brief description would 

 suggest, and of course differs in detail in different groups, 

 but the fundamental pattern is that just outlined. In 

 the wormlike sea cucumbers the canals are present in the 

 young, but lost in the adult. The sea urchin was 

 studied ages ago by Aristotle, and because of its spiny 

 surface he called it Echinus, or hedgehog. This name 

 is still used for the animal, and has become the basis of 

 the name of the phylum, Echinodermata meaning 

 "hedgehog-skinned" or "spiny-skinned." Aristotle 

 observed that the mouth and gullet of the sea urchin 

 (on the lower surface) are surrounded by a series of 

 elongated pointed plates, which serve for mastication. 

 The whole structure resembles a lantern, and is often 

 called "Aristotle's lantern." Reproduction is sexual, Aristotle's 

 but arms of starfishes, if removed with a portion of the ant< 

 disk, will develop into whole animals. 



2. Attempts have been made to understand the Psychology 

 psychology of echinoderms. Professor Jennings, work- starfish 

 ing on the coast of California, made many experiments 

 with the common starfish of that region. The animal 



