HETEROMORPHOSIS 147 
and the reader who is unacquainted with zodlogy 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 5-10cm. 
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 
U 
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 (ach, 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 bc, 
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 
Digitized by Microsoft® 
