PH YL UM ECHINODERMA 



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less than a year. The gonads increase in size with the approach 

 of the breeding season. Mature eggs are discharged directly into 

 the water. Sperm cells, discharged by sexually mature males, 

 are likewise set free in the water, where fertilization takes place. 

 Total, equal cleavage gives rise to a free-swimming ciliated 

 blastula. Following gastrulation, the rounded body changes 

 form (Fig. 84 A-D) and becomes known as the bipinnaria with a 

 number of ciliated lobes. In later development, the bipinnaria 



Fig. 84. — Development of starfish larvae. .4, early larva, oral view; B, the 

 same, lateral view; C, more advanced larva, oral view showing extent of ciliated 

 bands; D, bipinnaria larva; E, larva in brachiolarian stage. {Redrawn from A, 

 Agassiz) . 



becomes further modified in form and is known as a brachiolarian 

 larva (Fig. 84 E). Each of these larval forms is distinctly 

 bilaterally symmetrical, yet by an intricate metamorphosis the 

 radial symmetry of the adult starfish is superimposed. In some 

 asteroids, development has become modified, resulting in the 

 elimination or reduction of the free larval stages. In some 

 instances (Leptasterias hexactis), the female carries the eggs and 

 embryos in a brood pouch until the fully formed young are 

 able to shift for themselves. Powers of regeneration are devel- 

 oped in an extreme degree among the Asteroidea. 



