HETEROMORPHOSIS 



147 



and the reader who is unacquainted with zoology can best 

 picture the animal to himself by imagining a sack made up 

 of an elastic, contractile wall, the opening of which is sur- 

 rounded by tentacles. 



The animals which I obtained in Naples were 5-10 cm. 

 long. They are very common in the Mediterranean and are 

 especially adapted to physiological experi- 

 ments because they are very tough and 

 comparatively long-lived. The animal lies 

 buried in a mucous envelope in the sand. It 



FIG. 27 



FIG. 28 



FIG. 29 



only thrusts its head outside of the envelope to catch small 

 marine animals for its prey. The envelope is formed by a 

 secretion from the skin. 



I made diagonal incisions (acb, Fig. 27) into the middle 

 of a large number of such Cerianthi. After a few days 

 new tentacles begin to spring from the cut surface 6c, 

 which grow rapidly and correspond in form, color, and mark- 

 ing with the tentacles at the oral pole. I have never seen 

 even an indication of the formation of new tentacles at 

 the other cut surface, ac. Figs. 28 and 29 represent such an 

 animal eight days after an incision had been made. Tentacles 



