272 ECHINODERMS. 



is pushed out through holes in the middle layer to line 

 the branchial tentacles, and is infolded to form the mesen- 

 teries. Pass a slender probe into a branchial tentacle. 



Boil a portion of an arm in potash solution, to remove 

 the connective tissue which holds together the calcareous 

 plates. When these seem about ready to fall apart, wash 

 and separate them, to see how they are put together. 

 Find : 



1. A double row of ambulacral plates, meeting by their 

 ends at an obtuse angle to form the gable roof of the 

 ambulacral furrow. 



2. On either side a bordering row of adambulacral 

 plates. 



3. Other plates irregularly disposed in other portions of 

 the body wall. 



What is the shape of the plates of all these sorts, and 

 how are they put together? 



Between the walls of the alimentary canal and the body 

 walls is a distinct body cavity which the blood (hemo- 

 lympTi) occupies. The hepatic cseca and the reproductive 

 organs are extended laterally into it, as already noted. 



Life Process. Only in the details for carrying out 

 the vital processes do we find anything new or unusual in 

 the starfish. 



I. Nutrition. The digesting of a mollusk in its own 

 shell without having been swallowed is unusual ; but, when 

 we remember that digesting is mainly dissolving prepara- 

 tory to absorbing, we see that the difference is merely one 

 of position. Digested food passes directly into the blood 

 by osmosis, and is carried with currents set up by the feebly 

 pulsating circulatory apparatus. In the branchial tentacles 

 it is properly exposed for aeration ; and in all the tissues, 

 as ever, its constituents are selected by the individual 

 cells for use. 



