160 PRACTICAL ZOOLOGY. 



For the anatomy and development of the starfish and 

 sea-urchin, see Brooks' " Handbook of Invertebrate Zool- 

 ogy," Hyatt's " Common Corals, Hydroids and Echino- 

 derms" (No. V. of "Guides for Science Teaching"), 

 "Seaside Studies in Natural History," by E. C. and A. 

 Agassiz, Romanes' " Jellyfish, Starfish, and Sea-Urchins " 

 (Vol. XLIX. in the International Scientific Series). 



THE SEA-URCHIN. 



The requisites for this work are, cleaned skeletons, or 

 tests, alcoholic specimens, microscopic sections, etc., as in 

 the case of the starfish. 



THE CLEANED TEST. 



1. Observe the radial distribution of the parts around an 

 axis, at one pole of which is a large opening. 



At the opposite pole is a circular area composed of 

 several small plates, near the center of which is the 

 anal opening. 



2. Note that the test is composed of distinct pieces or 

 plates. Put one of the plates into a little dilute acid 

 and note what occurs. 



3. To make out the real nature of the skeleton, proceed 

 thus : 



a. Handle an entire decalcified specimen, i.e., one 

 from which the calcareous matter has been re- 

 moved by chromic or other acid. Observe that 

 the body walls and. spines are still present. 



