STRUCTURE OF STARFISH 



297 



It is likely that pigmented cells of the body cavity fluid act 

 as rudimentary red blood corpuscles ; the water vascular 

 system may help in aeration ; and the whole body is, of 

 course, continually washed with water. 



The " skin-gills " are said 

 to have an excretory function ; 

 for phagocytes, bearing waste, 

 seem to traverse their walls. It 

 may also be that excretion is 

 somehow concerned in forming 

 the carbonate of lime skeleton, 

 but facts are wanting. 



The sexes are separate, and 

 they are like one another, both 

 externally and internally. The 

 gonads develop periodically, and 

 lie in pairs in each arm. Each is 

 branched like an elongated bunch 

 of grapes, and is surrounded by 

 a " blood sinus." Each has a 

 separate duct, which opens on a 

 porous plate, between the bases 

 of the arms on the dorsal sur- 

 face. In Asterina gtbbosa, how- 

 ever, the eggs are extruded ven- 

 trally. In the same species 

 there is an interesting sexual 

 variability : many are first males 

 and then females (protandric), 

 others are simply hermaphrodites, 

 others seem exclusively of one 

 set. The eggs of starfishes are 

 fertilised in the water, and the 

 free-swimming larva is known 

 as a Bipinnaria or as a Brachio- 

 laria. 



Fig. 161. — Regeneration of 

 a starfish from a separated 

 arm — the so-called " comet 

 form." 



T.F., Tube-feet in the ambulacral 

 groove [A.G.) of the separated 



"■ arm, which is growing the other 

 four; M., the mouth; R.A., 

 the radial or ambulacral area of 

 one of the four new arms. 



Other Starfishes 



Parental care is incipient among Asteroids. A species of 

 Asterias has been seen sheltering its young within its arms ; 



