302 MEADFOOT AND THE STARFISH. 



which extends from the hollow in the centre, where 

 the mouth is seated, throughout its length, to the 

 point. Along the floor of this groove we should see 

 in the dead and dried animal four rows of minute 

 perforations, running lengthwise. We cannot dis- 

 cern them directly during the living activity of the 

 starfish, because the crowding sucker-feet conceal 

 them. Each of these suckers is a tube of delicate 

 membrane, a continuation of the common skin ; and 

 its interior accurately corresponds with one of these 

 perforations in the skeleton. The tip of the tube is 

 expanded into a broad circular flat disk, which retains 

 its form, owing to its being strengthened by an inter- 

 nal horizontal plate of calcareous glass, which has a 

 broad hole in its centre. The use of this you will 

 presently discover. 



If we were to dissect this animal, we should find, 

 on the interior surface of the semi-crustaceous integu- 

 ment of the arm, a little globular bag of similar trans- 

 parent membrane, on each aperture, which opens into 

 the cavity of the globe, just as on the outer side it 

 opens into the tube. Thus there is a free intercom- 

 munication between the globose sac on the inside 

 and the sucker-tube on the outside, through the tiny 

 perforation in the crust. The interior is filled with a 

 clear fluid, scarcely differing in its nature from sea- 

 water. The globular sac within and the tube with- 

 out are both composed of highly contractile tissue, 

 under the control of the animal will. 



We must bear in mind that we have been consider- 



