38 STUDIES OF ANIMAL LIFE. 



C Exoskeleton. 



An exoskeleton is any hard substance developed in or 

 upon the skin of an animal. 



1. Identify the small plates (ossicles) which make up 



the skeleton of the starfish. (Dem.) 



2. Examine the hard projections (spines). What is 



their use ? 



3. How do the spines along the edges of the ambula- 



cral grooves differ from the other spines ? What 

 is the reason for this difference ? 



4. Why is the skin between the ossicles so tough and 



leathery ? 

 6. Identify among the spines on the aboral surface soft 



tapering structures (aboral tentacles or dermal gills), 



used in breathing. (These show much better if 



the specimen is under water.) 

 6. Observe a thread-like ridge along the middle of 



the groove. This indicates the position of a radial 



nerve. 



D, Ambulacral Feet and Madreporlte. 



Identify tube-like projections (ambulacral feet) along 

 the ambulacral groove. 



1. How many longitudinal rows of feet do you find in 



each groove? 



2. Are the feet in adjacent rows opposite or alternate ? 



(Examine also a dried specimen from which the 

 ambulacral feet have been removed.) 



3. Observe the pinkish sensory spot at the distal end of 



the groove. This is called an " eye-spot." 



4. What is the appearance of the madreporite under a 



lens? 



