'BRANCH ECHINODERMATA : STARFISHES, ETC. 113 



already been mentioned as the respiratory caeca (see 

 p. 109). Surrounding the stone canal is a thin mem- 

 branous tube, and within it and by the side of the stone 

 canal is a soft tubular sac. The function of these organs 

 is not certainly known. 



Work out the nervous system; note, as its principal 

 parts, a nerve-ring about the mouth, and nerves running 

 from this -ring beneath the radial canals along each arm. 



Life-history and habits. The starfishes are all marine 

 forms. They hatch from eggs, and in their early stages 

 are very different in appearance from the adults. At first 

 they are bilaterally symmetrical, their radial symmetry 

 being acquired later. Thousands of eggs and sperm-cells 

 are extruded into the sea-water, where fertilization and 

 development take place. The young swim freely in the 

 open sea, feeding on microscopic organisms, and then 

 undergo very radical changes in the course of their 

 development. The adults are for the most part carniv- 

 orous, feeding on crabs, snails, and the like. The live 

 prey is surrounded by the extruded stomach which secretes 

 fluids that kill it, after which the soft parts are digested. 

 (See general account of the life-history of Echinoderms 

 on p. 1 19.) 



THE SEA-URCHIN (Strongylocentrotiis sp.) 



External Structure. TECHNICAL NOTE. If fresh or alco- 

 holic specimens or even the dry "tests" of the sea-urchin (fig. 20) 

 are to be had, the general characteristics of the external structure 

 can be made out. 



How does the external surface of the sea-urchin differ 

 from that of the starfish ? Can you find the very long 

 tube-feet ? Where is the mouth-opening ? With what is 

 it surrounded ? Each tooth is enclosed in a calcareous 

 framework. The whole structure is known as " Aristotle's 

 lantern. ' ' 



