REGULATION OF HARENACTIS ATTENUATA. I 5 



tion to altered conditions. But the question as to why the 

 elongated form is characteristic of animals in nature, /. e., in bur- 

 rows, has not been considered. Evidently this shape is not inde- 

 pendent of external conditions, for when we alter the conditions 

 the shape changes. But why should not the animal take the 

 form of a rounded sac or a flattened disc in the sand as well as 

 elsewhere ? It is of course easy to say that the form is inherited, 

 but besides being no explanation, this is obviously not true in 

 this case. 



In my opinion it is impossible to account for the shape of the 

 adult Harenactis without considering its behavior. In the first 

 place, the burrows of the species are almost invariably vertical 

 or nearly so, and the behavior of the animals outside their bur- 

 rows indicates a reaction to gravity. If we suppose that a young 

 animal entering the sand has sooner or later brought its axis 

 approximately into coincidence with the direction of gravity, 

 muscular elongation ot the body will undoubtedly occur more 

 frequently thereafter in this direction than in any other. This of 

 itself is without doubt a factor in determining the shape of species 

 without hard parts. After growth in the axial direction has 

 begun and the body is more or less elongated, there is less re- 

 sistance in the sand to growth in this direction than in any other. 

 Moreover, the aboral end is not only an organ of attachment but 

 a burrowing organ. So long as the end comes into contact only 

 with the soft sand, the animal tends to extend and to force the 

 end deeper and deeper. Thus these different factors combine to 

 bring about greater growth in the longitudinal than in other 

 directions. As was noted above (pp. 23), the lengths of the ani- 

 mals differ very greatly according to the depth of the layer of 

 sand in which they live. The longest individuals I have ever 

 found were in sand which was almost free from stones or broken 

 shell for a depth of 40-50 cm. and many of the animals living 

 there had attained this length and were attached. But these 

 animals were not simply more elongated than those living under 

 other conditions, they were actually larger. The diameter was 

 fully as great as that of the largest specimens found under other 

 conditions and in the contracted condition it was clearly evident 

 that the individuals from the deep sand possessed much greater 



