FORM REGULATION IN CERIANTHUS. 243 



sand with the oral end and tentacles protruding. In this position 

 they secrete about the body a mass of tenacious slime in which 

 sand-grains and other foreign bodies become imbedded, the whole 

 forming a tube into which the animal may withdraw. Loeb has 

 given an interesting account of the geotactic reactions of these 

 animals and my incidental observations upon this point confirm 

 his. He has also described a number of experiments concerning 

 the external conditions which determine the tube-formation. 



When specimens are kept in aquaria without sand they creep 

 about to a considerable extent, often climbing the sides. When 

 left undisturbed they usually orient themselves as Loeb has noted, 

 so that the oral end of the body is directed upward, even if this 

 position necessitates the bending of the body at right angles. In 

 the jars they secrete a considerable amount of slime and often 

 form tubes along the sides or bottom, in which they remain for a 

 longer or shorter time. When handled or otherwise irritated, and 

 especially when cut, the secretion of the slime is especially, 

 rapid. 



When undisturbed, the body and tentacles are usually more or 

 less distended with water and the body-wall is always tense. In- 

 deed, as will be shown later, complete extension of the body and 

 erection of the tentacles is impossible without internal water- 

 pressure, /. c. t without water in the enteron. If the body of a 

 distended animal is opened quickly by a small cut the water 

 issues with considerable force, and when an individual is made to 

 contract rapidly by sudden stimulation the water squirts from the 

 aboral pore with great force. The inability of the animal to ex- 

 tend to its full length without the aid of water-pressure is due to 

 the absence of circular muscles in the body-wall. Extension is 

 passive, not active. The longitudinal muscles are powerful and 

 under strong stimulus the body may be torn apart if the ends are 

 fastened. 



It was found that the animals could be kept alive for months 

 without other food than the small forms and organic particles 

 which the water might contain, and in the present series of ex- 

 periments no attempt was made to give them food. Of course in 

 the early stages of regeneration and throughout many of the ex- 

 periments the pieces were unable to take food ; moreover, the 



