CHAPTER XVIII 



BRANCH ECHINODERMATA : STARFISHES, 

 SEA-URCHINS, SEA-CUCUMBERS 



STARFISH (Asterias sp.) 



TECHNICAL NOTE. The species of Asterias are widely dis- 

 tributed on both coasts of the United States and may be procured 

 on almost any rocky shore at low tide. Teachers in inland schools 

 can obtain preserved material from the dealers mentioned on p. 

 453. Most of the specimens should be placed in alcohol or 4$ 

 formalin. If fresh material can be had it is well to place at least . 

 one specimen for each student in a 20% solution of nitric acid in 

 water for two or three hours, when all of the calcareous parts will 

 have been dissolved, and after a thorough washing the specimen 

 will be ready for use. 



External structure (figs. 18 and 19.) In a fresh 

 specimen or one which has been preserved in alcohol or 

 formalin note the raying out of parts of the body from a 

 common centre. This is characteristic of the body or- 

 ganization of all Echinoderms, and is known as radial 

 symmetry. The lower surface of the body is called the 

 oral (because the mouth is on this surface), while the 

 upper is called the aboral surface. The central part of 

 the body is called the disk. Note on the aboral surface 

 of the disk a small striated calcareous plate, the madre- 

 porite or madrcporic plate. In the middle (or very 

 nearly in the middle) of this surface of the disk there is 

 a small pore, the anal opening. The entire aboral sur- 

 face as well as a greater part of the oral side is thickly 

 studded with the calcareous ossicles of the body-wall. 

 These ossicles support numerous short stout spines ar- 

 ranged in irregular rows. Note that some of the ossicles 



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