SEA-MATS AND SHELLY CORALLINES 79 



Moreover, the globular chambers show signs of life; 

 their front doors suddenly open, gape widely, and then 

 shut with a snap; and presently this opening and shutting 

 is repeated. The meaning of this action you will better 

 understand when we see analogous organs in another form 

 of the same class of animals. Meanwhile, I will just point 

 out a beautiful though minute proof of design in a point 

 of the structure of the cells connected with these pearly 

 chambers. If you look closely, you will see that the 



LEAFY SEA-MAT. 



(A portion magnified.) 



spines of the margin are not found on those cells tha* 

 carry the pearls; and, moreover, that they are also want- 

 ing on the approximate edges of the two cells that lie be- 

 hind every such pearl-bearing cell. Now the reason of 

 this omission is obvious. The spines projecting obliquely 

 would interfere with the gaping of the door; and hence 

 they are invariably absent there. 



I happen to have in my aquarium a living individual of 

 another species belonging to the same class, and agreeing 

 with this in all essential particulars of structure, though 

 widely different in form. The difference, however, is 



