14 C. M. CHILD. 



out sand the enteron never contains anything like the amount of 

 water that is present when they are fully extended in the burrow. 

 In the burrow the whole surface of the body-wall except the oral 

 end is supported by the walls of the burrow, and so does not 

 support the pressure of the enteric water. The oral region and 

 the tentacles are evidently adapted to support a high pressure. 

 Outside the burrow, however, the body-wall supports the whole 

 pressure and one of several results is to be expected : first, disten- 

 tion may increase to a point where pressure of the body-wall upon 

 its contents is so great that no more water can enter or where 

 water passes out as rapidly as it enters ; second, rupture of the 

 body-wall may occur ; third, a regulatory increase in strength or 

 regulatory growth of the body- wall may occur ; fourth, the en- 

 trance of water may be regulated by some mechanism of corre- 

 lation. There is every indication that such a mechanism is present 

 in the actinians, for distension of the body-wall beyond a certain 

 point undoubtedly results in regulation in some degree of the 

 quantity of water entering the body, or else in opening the outlets. 

 Evidently such regulation occurs in Harenactis for the internal 

 pressure in animals outside the burrows never approaches that 

 which frequently exists within the burrow. The decrease in 

 length of the body, the atrophy of the tips of the tentacles and 

 the partial atrophy of the body- wall, the shortening of the retrac- 

 tor muscles and mesenteries are all very evidently consequences 

 of the decrease in distension. As in Ceriantluis (Child, '04, a, b, 

 c, d, '05, '08), so here, the body-wall, the tentacles, the muscles, 

 and mesenteries cannot maintain their form except under a cer- 

 tain degree of tension or stretching. When this decreases atrophy 

 occurs to a greater or less extent and more or less change in 

 shape may result according to conditions. 



The broad foot- region of some individuals (Fig. 16) is, I be- 

 lieve, merely an indirect result of the change in shape which 

 makes contact possible over a large area. Undoubtedly a broad 

 foot-region would appear normally or occasionally in Harenactis 

 if conditions existed within the burrow which permitted con- 

 tinued contact between a large aboral area and a solid substratum. 



The transformation from the " normal " elongated form to the 

 short, broad "sessile" shape is very evidently a regulatory reac- 



