312 



COLLEGE ZOOLOGY 



Figure 197. Four-rayed starfish is regenerating one new ray which appears as a small bud 

 growing from the disk. (Courtesy of N.Y. Zoological Society.) 



The anatomy of the adult and the structure 

 of the larvae, however, show that these phyla 

 really occupy widely separated positions in 

 the animal kingdom. The adult echinoderms 

 cannot be compared with any other group 

 of animals, and we must look to the larvae 

 for signs of relationship. The bilateral larva 

 is either a modification for a free-swimming 

 life or an indication of the condition of its 

 ancestors. The latter view is accepted by 

 most zoologists. The ancestors of echino- 

 derms were doubtless bilateral, wormlike 

 animals which became radial and took up 

 sessile habits secondarily, and present-day 

 free-living echinoderms are probably derived 

 from a fixed ancestor whose symmetry they 

 still retain. 



RELATIONS OF 

 ECHINODERMATA TO MAN 



The value of echinoderms in their rela- 

 tions to other lower animals is problematic. 

 Apparently, however, they are of consider- 

 able importance to man. In the Orient, sea 



cucumbers are dried in the sun and sold as 

 beche-de-mer or trepang for use especially 

 in soup. The gonads of the sea urchin and 

 the eggs of the starfish are also eaten in 

 certain tropical regions. 



The dried skeletons of echinoderms have 

 been crushed and used as a fertilizer be- 

 cause of the high calcium and nitrogenous 

 content. 



Starfish are very destructive in oyster beds 

 since they succeed in pulling open the 

 shells and eating large numbers of these 

 bivalves (Fig. 192). A starfish has been ob- 

 served to eat 10 oysters or clams in a day. 

 Two control measures are in general use 

 at the present time: (1) in some regions a 

 moplike tangle of threads (Fig. 198) is 

 dragged across the oyster beds and the star- 

 fish that grab onto these with their pedicel- 

 lariae are removed from the water and 

 destroyed; and (2) the most efficacious 

 method of killing starfish is to spread quick- 

 lime over the oyster beds in a strength that 

 is harmless to the oysters but is death to 

 the starfish. 



