196 



GENERAL ZOOLOGY 



soft parts of the body are shielded from small predaceous 

 animals. 



If a starfish meets with an accident and has one of its 

 arms hurt, the injured appendage is cast off near its base and, 

 after a time, a new one grows out. The body may be cut 

 in two and each of the parts can regenerate what it lacks. 

 By careful experiments it has been found that Asterias for- 

 besi is able to form a complete body if left with the disc and 

 one arm (Fig. 80, B). Professor Mead kept a separate 

 arm alive for three months, but it did not regenerate any- 

 thing, although it was apparently in good condition and 

 crept about actively. Some years ago starfishes were com- 

 monly dredged up, cut in two, and thrown back into the 



FIG. 81. Development of starfish. A, egg and sperm cells; B, zygote; 

 C F, cleavage stages; F, morula; G, section through gastrula stage; H, ciliated 

 bilaterally symmetrical larva; a, anus; m, mouth; I, young starfish growing out 

 from side of bilaterally symmetrical larva. 



ocean. The oyster fisherman supposed they were killed 

 by such treatment, but it of course only increased the num- 

 ber of the pests. 



Race Preservation. Starfishes are dioecious animals and 

 both the males and females shed their ripe sexual cells into 

 the ocean. The sperm cells swim to the ova, fertilize them, 

 and development passes through the stages characteristic 

 of all metazoans (Fig. 81, A-G). After gastrulation (G) 

 a bilaterally symmetrical larva is formed, which is known 

 as a brachiolaria (H). It has a mouth (m), digestive tube, 



