124 COELENTERATES 



furrow and see if a channel can be found through which 

 the water runs. 



(lo) Make out how the ampullae are connected with 

 the tube feet. Remove an ampulla and its corresponding 

 foot and make drawing. Make drawing of cross section of 

 ray showing all the points you have found. 



(ii) Scrape the aboral side of a ray to see if there is 

 anything of a skin or of a membraneous structure at the 

 base of the spines. Do the spines seem to be attached to 

 the skin? Do the aboral tentacles come off with the skin? 

 Do you find a skin under the hard plate ? 



The hard part is probably developed from the membrane. 

 (12) Make a drawing of the water vascular system in 

 the disk and in one ray. 



Sqmmary. — The animals of this branch are not much 

 sought after for food by other animals in the ocean. The 

 spiny skin after which the branch is named is generally 

 modified into some form of defense. What is the nature 

 of this defense in the forms you have studied ? Some forms, 

 as the sea urchin, act as scavengers, but otherwise they are 

 of no value. Do you notice any general plan in the structure 

 of all? Notice the forms, sizes, and relative freedom of 

 motion of the spines in the various forms. 



Classification. — The relationships of the common forms may be 

 seen by the following outline of the Branch Echinodermata : 

 Class I. Asteroidea. Starfish. 

 Class 2. Ophiuroidea. Serpent Stars. 

 Class 3. Echnoidea. Sea Urchins. 

 Class 4. Holothuroidea. Sea Cucumbers. 



