ASTEROIDEA 45 



ciliated, are supposed to have a respiratory function ; it appears 

 that the cribriform organs also act as a sieve to get rid of the 

 coarser bottom material (the Porcellan-asterids being mud-eaters). 



The mouth is on the underside, in the middle, where the 

 ambulacral grooves join. There are no teeth, but the innermost 

 adambulacral spines are bent over the mouth edge, forming thus 

 a sort of mouth armature. The stomach is large, sac-shaped, 

 filling up nearh^ the whole disk. A pair of large, branched 

 jDyloric sacs (liver) proceed from the stomach into each arm. 

 As a rule there is a short, thin rectum, from which some small 

 rectal caeca may proceed. The anal opening, which is small 

 and often difficult to observe, is situated in the centre of the 

 disk. It is wholly wanting in some forms. In the genus Luidia 

 both rectum, rectal caeca, and anal opening are wanting. 



The genital organs are bush-shaped, branching bodies, attached 

 to the base of the rays, one on each side ; when they contain ripe 



>-. - 1 : 



.rw-. 



Fig. 27. — Side view of Hyphalaster inermis, showing cribrifonn organs. 

 (After Sladen.) 



sexual products they may extend far out into the rays. In 

 some forms {e.g. Luidia) there is a series of small genital organs 

 extending along each side of the arms, nearly to the point. The 

 genital openings are situated at the base of the rays, sometimes 

 on small papillae ; they are generally difficult to observe. Some 

 forms have several openings to each genital organ ; where there 

 are series of genital organs, each single organ has its special 

 opening. 



The nervous system is not very highly developed ; the best 

 developed part is the ambulacral nervous system, which is seen 

 as an elevated median ridge in the ambulacral furrows. 



The colour, which is often a beautiful red or violet, always 

 disappears completely on preservation in alcohol. In formalin 

 the colour will, in some forms, keep very beautifully for a longer 

 period, and it will often keep well in dried specimens, when these 

 have been preserved for some time in formalin. 



Some of the sea-stars found in the British seas {Henricia, 

 Pteraster, LeiUasterias Mulleri) protect their brood ; in these 

 cases the development is direct. In Solaster, and probably 



