268 C. M. CHILD. 



crease in size either locally or at all points, the result being in 

 the first case increase of pressure and either stretching of other 

 parts of the sac or loss of water through the pore or mouth, 

 and in the second case always loss of water, commonly through 

 the pore, and reduction in size of the whole body. While this 

 sac has the power of contracting and forcing water out through 

 certain openings, it has not the power of expanding actively and 

 so drawing water in. 



Since water under pressure is normally present in the enteron, 

 and since after collapse due to contraction the pressure is soon 

 reestablished, means must exist by which water can be forced 

 into the enteron against pressure. In my first paper (Child '03^) 

 mention was made of the fact that regenerating pieces closed at 

 both ends by their membranes of new tissue become distended 

 with water in the course of a few days. It was suggested that in 

 such cases water diffuses through the body-wall in consequence 

 of the accumulation within the enteron of certain soluble products 

 of metabolism. It is also possible that water may be secreted 

 into the enteron together with these substances. To what degree 

 the distension in normal animals is caused in this manner cannot 

 be definitely ascertained, but it is certain that the rapid accumula- 

 tion of water in the enteron of the normal animal after this has 

 lost all or nearly all of its contents in consequence of violent con- 

 traction cannot be due to diffusion or secretion. In such cases 

 the animal frequently regains its usual degree of distension within 

 fifteen or twenty minutes, whereas distension by diffusion in re- 

 generating pieces requires several days. 



The few observations which have been made upon the sipho- 

 noglyphes in actinians indicate that the function of these struc- 

 tures is the production of currents through the stomodaeum. 

 Hickson ('83) found that in certain Alcyonaria, which possesses 

 only one siphonoglyphe, the current was inward or downward 

 along the siphonoglyphe and outward on the other portions of 

 the stomodseal surface. It is probable further, according to 

 Hickson, that the inward or downward current is continued 

 along the free edges of the mesenteries for a greater or less dis- 

 tance below the siphonoglyphe, while on other mesenterial mar- 

 gins the cilia beat in the reverse direction, like those of the gen- 

 eral stomodaeal surface outside the siphonoglyphe. 



