198 



METAZOAN PHYLA 



the use of others. One animal was trained in 180 lessons, ten on each of 

 eighteen days, so to use certain rays; after an interval of seven days and 

 when left to its own initiative it was still using them. This type of 

 action acquired as a result of the repetition of an act is known as a habit. 

 228. Reproduction. — Both sperm cells and egg cells are set free in 

 the water, where fertilization takes place. The eggs undergo total and 

 equal cleavage, after which follows a typical embryogeny, including the 

 development of a hollow blastula, a gastrula formed by invagination, 



and a triploblastic embryo. A larva 

 known as a bipinnaria (Fig. 106) 

 is produced which is bilaterally 

 symmetrical but which gradually 

 metamorphoses into the radially 

 symmetrical adult. 



229. Regeneration and Autot- 

 omy. — The starfish has a consider- 

 able power of regeneration. If the 

 disc is deprived of all its rays it will 

 regenerate them all, and a single ray 

 with only a portion of the disc will 

 regenerate the whole animal (Fig. 

 107). 



The starfish also possesses the 

 power of autotomy. Ordinarily the 

 part dropped off is regenerated. 

 This ability serves as a safeguard to 

 the animal which, if it finds itself 

 caught by one or more rays, can 

 simply drop them off and make its 



Fig. 107. — Linckia guildingii Gray, one . 



of a group of starfishes noted for their 230. Economic Importance. — 



regenerative ability. A to c three speci- gtarfishes are ofeconomic importance 



mens showing regeneration oi the whole 



animal from one ray. In such a case five Only as they are enemies ot oyster 



arms are regenerated making six altogether, fighermeu Or aS they destroy clamS 

 five being the normal number. When the ... i,- u 



disk is uninjured only the rays lost are and other marine animais which 



regenerated, as in D. Natural size. serve aS human food. Oysters live 



adherent to solid objects lying upon the bottom of the sea in areas known 

 as oyster beds. Starfishes come upon these oyster beds and destroy 

 the oysters in the manner already described (Sec. 226). Owners of oyster 

 beds formerly were in the habit of using drags made of frayed rope ends 

 which they hauled over the beds behind a vessel and in which the star- 

 fishes became entangled. The drag with its starfishes was then hauled 

 upon the deck. The starfishes were chopped to pieces and thrown back 

 into the water, but since the pieces were capable of regeneration this 



