112 G. H. PARKER 



this organism, I shall have occasion to refer to only two. These 

 both lie in the plane of bilateral symmetry. One, which I shall 

 call the primary axis, is the chief axis of the cylinder which the 

 actinian typifies; the other, which I shall call the secondary axis, 

 is perpendicular to it. 



II. SECONDARY AXIS AND DIRECTION OF LOCOMOTION 



In bilateral animals the chief or primary axis so commonly co- 

 incides with the direction of locomotion that it is natural to ask 

 whether a like relation exists in the actinians, for, as is well 

 known, many of these animals exhibit bilateral symmetry in- 

 ternally even though their parts seem to be radially disposed 

 externally. The four species studied were all sessile rather than 

 locomotor. Metridium crept rarely and slowly (Fleure and 

 Walton, '07, p. 218, note that Metridium dianthus may move 

 as much as 7 inches in a day), Sagartia moved frequently but 

 still slowly, Condylactis with greater freedom and speed, and 

 Actinia most freely of all. 



As a preliminary step specimens of all these species were 

 watched and when creeping was shown by any of them, a rec- 

 ord was made of the direction of locomotion in relation to the 

 secondary axis of the animal as indicated by the plane in which 

 the mouth was elongated. Thus a large brown specimen of 

 Metridium, whose pedal disc had a diameter of about 7 cm., was 

 observed to creep up the vertical side of a glass aquarium for a 

 distance of some 10 cm. and during the whole of its course the 

 secondary axis was approximately at right angles to the direction 

 of locomotion. In another instance a specimen crept upwards in 

 a jar over a stretch of about 9 cm. with the secondary axis coin- 

 cident with the direction of locomotion. In a specimen of Con- 

 dylactis whose pedal disc measured about 13 cm. by 8 cm., the 

 direction of locomotion also coincided with the secondary axis. 

 In Sagartia a specimen with a pedal disc measuring 15 mm. by 

 8 mm. was seen to creep over a distance of 4 mm. with this axis 

 in exact agreement with the direction of motion, though in a 

 second specimen the secondary axis was observed to be almost 

 precisely at right angles to the direction of creeping. In six 



