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234 TEXTBOOK OF ZOOLOGY 



nerve which extends from an anal nerve ring. Branches of these 

 nerves extend to the numerous nerve cells distributed in the epi- 

 dermis above the nerve cords. The pigment eyespots at the tips 

 of the arms are photosensitive and sensitive to touch. The pedi- 

 cellaria and tube feet are also sensitive to touch. There is little 

 centralization except in the oral ring and radial cords, still there 

 is sufficient centralization for the necessary coordination exercised 

 by the animal. 



Reproduction and Life Cycle 



The starfish is dioecious ; i. e., the sexes are separate. The repro- 

 ductive systems of the two are similar and each consists of five 

 paired gonads lying in the cavity of the rays beside the pyloric 

 caeca. They open to the exterior by pores in the angles between • 



arms. Mature eggs produced in ovaries of females and mature : 



spermatozoa discharged from testes of males are freed in the ocean 

 water where they unite in fertilization. Total, equal cleavage is the 

 type of division which follows fertilization, and this finally gives rise 

 to the many-celled, free-swimming, ciliated hlastula. The wall of this 

 infolds to form a gastrula. Following this the rounded body becomes 

 somewhat elongated and lobed. Ciliated bands develop over its sur- 

 face and it is known as hipinnaria. This larval stage has bilateral 

 symmetry, and the larva swims about near the surface for weeks 

 by the aid of its ciliated bands. A later modification of the hipin- 

 naria in which there are several extended symmetrical processes, 

 is known as the hrachiolarian stage (Fig. 128). Following this condi- 

 tion is a metamorphosis during which many processes are formed, and 

 the radial symmetry superimposes the bilateral. The presence of the 

 bilateral symmetry in these larval stages seems to indicate that the 

 ancestors of echinoderms were likely animals with this type of 

 symmetry. 



Regeneration and Autotomy ■! 



Regeneration is the name applied to the power some animals have 

 to replace mutilated or lost parts. The starfish has this phenomenon 

 quite well developed with regard to its arms. Any or all of the 

 arms of the starfish may be lost and the missing parts regenerated. 

 (Fig. 367.) An arm with a small portion of the central disc will 

 regenerate the missing parts under favorable conditions. A muti- 

 lated arm or one caught in the grip of some enemy may be cast off 



