2O4 HARRY BEAL TORREV. 



it was uncovered about half each day by the falling of the tide, 

 during which periods its tentacles were completely retracted, con- 

 trary to the custom of such typical sand dwellers as Harenactis 

 attenuata? and another species of Sagartia from San Pedro as yet 

 undescribed. It may live permanently submerged, however, and 

 thrives in aquaria. 



Though ordinarily attached to a solid substratum, it is occa- 

 sionally found free on the sand. Its powers of locomotion are 

 considerable. By means of multicellular amoeboid processes of 

 the foot disk, readily seen with a hand lens at the edge of the 

 disk, it is capable of creeping more than an inch in an hour. The 

 polyps often leave the clam to which they are attached when 

 placed in an aquarium, especially when they are on the lower 

 valve. They may occasionally creep along the surface of the 

 water, hanging from the surface film. 



The inverted position is not long retained, however, for 5. 

 davisi has a marked tendency to assume as erect a posture as its 

 situation will permit. An example of this tendency is provided 

 in the case just mentioned, of the relatively greater haste of the 

 polyps in leaving the lower than the upper valve of a clam lying 

 on the aquarium floor. Moreover, the axes of polyps clinging to 

 the vertical sides of the aquarium are either perpendicular to the 

 sides or bent upward. They never bend downward if the polyps 

 are submerged. 



The orientation of 5. davisi is, then, partly a result of geotro- 

 pic stimulation. The same may be said of the locomotion of the 

 species. There is a definite tendency of the polyps on the walls 

 of the aquaria to collect near the surface, although the aquaria 

 may be sealed jars completely filled with water, or furnished with 

 green water plants evenly distributed (precautions against the 

 possible influence of oxygen at the surface). The polyps on the 

 floor of the aquarium, if it be horizontal, move about but little, 

 and when they do, sporadically and without certainty of direc- 

 tion. When by chance, however, they reach the angle made by 

 the floor and a side of the aquarium and begin an ascent, there 

 is never a retrograde movement, seldom a halt, until they draw 

 near the surface. This locomotor geotropism is especially inter- 



1 Torrey, 1901. 



