4 8 



C. M. CHILD. 



tent, the usual degree of distension cannot be maintained in this 

 wajrand in the absence of the functional stimulus produced by 

 distension atrophy occurs. 



In the case of Harenactis the occurrence or non-occurrence of 

 atrophy depends upon the conditions of the experiment and the 

 result can be controlled. 



The possibility of experimental control in Harenactis depends 

 on the position of the mesenterial ostia in the cesophageal region. 



The ostia are readily seen in anaesthetized 

 specimens opened under water. Their ap- 

 proximate position is indicated by the small 

 dotted circles on each side of the oesophagus 

 in Fig. i. This figure represents an individual 

 in the extended condition characteristic of the 

 normal habitat, a vertical burrow in the sand. 

 When the animals are kept without sand ex- 

 tension is never as great as in the burrow and 

 the animal gradually becomes incapable of 

 any considerable extension. 1 Under these 

 conditions the ostium appears to lie much 

 nearer the distal end than in Fig. i. Each 

 mesentery possesses an ostium, consequently 

 the inter-mesenterial chambers are in direct 

 communication with each other about the 

 whole circumference of the cesophageal region. 

 I have been unable thus far to discover the 

 ostia in Cerianthus or to find references to them 

 in the literature, so far as it is accessible to me. 

 But whether they are present or not my ex- 

 periments show very clearly that distension of 

 the region directly distal to a lateral incision 

 in the cesophageal region does not occur. The 

 most casual examination of specimens after healing of the wound 

 produced by such an operation is sufficient to show that this 

 region is either completely collapsed or only very slightly dis- 

 tended. 



FIG. i. 



'Child, "Form Regulation of Harenactis attenuata in Altered Environment," 

 BIOL. BULL., Vol. XVI., No. i, 1908. 



