94 



STARFISH, SEA URCHIN, BRITTLE STARS 



by eggs and the sexes are separate. The ovaries of the 

 female are within the body cavity at the bases of the arms. 

 There is one pair for each arm. The spermaries of the 

 male are in a similar situation. When the ova are mature, 

 they are extruded into the sea through small pores in the 

 body walls at tlic bases of the arms. In the sea they are 

 fertilized by the sperms and finally hatch into forms verj' 

 different from the adults. It is also an interesting fact 

 that the young are bilaterally symmetrical. The yoimg 

 feed on microscopic organisms and, as they grow, undergo 

 radical changes imtil finally they assume the form of the 

 parent starfish. 



Other Echinoderms 



Not all starfish have just five arms. Indeed, abnormal 

 specimens of the common starfish are sometimes found 

 with four or six arms. But there are some species that 



normally have more than 

 five arms. One species 

 has as many as thirty. 

 Again, the members of 

 one genus have sLx arms, 

 an irregular number. 

 Not all starfish are like 

 the one described in 

 regard to the central 

 ilisk. In some, the disk 

 is more plainly marked 

 off and is more distinct, 

 while in others the 

 spaces between the 

 arms are filled up nearly 



Fig. 46. — Ventral surluco of a kind of star- 

 fish in which the disk extends much farther 

 out than in the common starfish. 



