ECHINODERMA TA 



63 



suckers at their tips ; then, when firmly fixed, they contract 

 and thus draw the body forward in the required direction ; 

 they then become detached and the same performance is 

 repeated. 



To understand more clearly the action of these tube-feet, 

 a knowledge of the internal structure of the creature is 

 necessary. The best way of gaining this is by a simple 

 dissection of a specimen. 1 



T , . Such a dissection will first of all disclose the 



Internal 

 Structure, alimentary canal leading from the rather wide 



Alimentary mouth on the ventral side to a very minute 

 Canal. aperture, the anus, near the centre of the dorsal 

 surface (Fig. 31, a). This alimentary canal is by no means a 

 uniform tube, but is modified into widely differing portions 

 with varying functions. The mouth leads by a very short 

 gullet into a large round stomach, just above which, at the 

 base qf the five arms, five large branches are given off. 

 Each branch divides to form two long closed digestive sacs 

 which lie in the cavity of the corresponding arm (Fig. 31, /). 

 Two other small processes are given off from the intestine 

 above the stomach near 

 the anus, one of which 

 is shown in Fig. 31. 



The star- 

 Food. n , * , 



nsn teeds on 



small sea -snails, also on 

 oysters, scallops, mussels, 

 and other bivalves. The 

 fact that the latter forms 

 may be too large for it to 

 swallow is in no way a 

 hindrance, for in such a 

 case the starfish first 

 Crawls right on to the FlG - 32. View of a Starfish (Echinaster) 



top of the bivalve, then 

 pulls asunder the two 



L , -i . i 



valves with its suckers, 

 and finally protrudes its stomach right through its own mouth, 

 turning it inside out and spreading it over its prey so that it 

 partially digests its food outside of its body instead of inside ! 

 1 See practical notes, page 69. 



"" 



i The madreporite. 



