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.^ 

 Internal Such a dissection will first of all disclose the 



Structure, alimentary canal leading from the rather wide 



Alimentary mouth on the ventral side to a very minute 

 *°* ■ 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 of 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, V). 



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- 

 fish feeds 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 Fig. 32.— View of a, Starfish (KchinasUr) 



top of the bivalve, then 

 pulls asunder the two 

 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 ! 

 ' See practical note.s, page 69. 



devouring a Mussel. (From the Cam- 

 bridge Natural History. ) 



u, The madreporite. 



