BILATERAL TENDENCIES IN PYCNOPODIA HELIENTHOIDES. 243 



depth (see topic on distribution, p. 233) to a depth of about 

 three feet, will move down to a depth of at least five feet. Then 

 the irritability limit ranges between the pressure of 2.5 feet and 

 5 feet of sea water. The average depth of the 75 specimens 

 referred to above, on an area of about 30,000 square feet, was 

 4.76 feet. The minimum pressure in this irritability limit, to 

 which Pycnopodia is adapted, is seen to be about 1.13 Ib. to the 

 square inch. 



An irritability limit may seem improbable when it is considered 

 that this starfish has been found on piles above water and on dry 

 tide-flats during low-tide. Either of these cases is very extra- 

 ordinary, in that tidal recession on tide-flats is sometimes great 

 enough even to catch swiftly swimming fishes. On piles onto 

 which the gluttonous starfish has been lured by the presence of 

 barnacles, the tidal recession is indeed the slowest possible, but 

 in this case the star is in a very awkward position to move. It 

 has already been stated that the righting movements are almost 

 invariably toward the same side. This is also the case relative 

 to distance-movements. One end is, much more so, than in any 

 other starfish, the anterior end; Pycnopodia has a decided bi- 

 laterality in this regard. That is, it is used to move with the 

 same side as anterior end. 



When it moves the rays are curved at the ends, the eye-spots 

 wide open. The second pair of primary posterior rays is pulling 

 along with the odd posterior primary ray; the sucker-feet are 

 engaged in a lively and continuous march. The third pair of 

 primary posterior rays is occasionally lifted foreward spontane- 

 ously, while the first pair of the post-larvel rays which is at right 

 angle to the anterio-posterior rays, when the star is at rest, is 

 lagging somewhat behind. During such movements, this an- 

 terior end is decidedly used as such, so that when the star en- 

 counters obstacles which it may not crawl under or over, it 

 merely, as an ordinary bilateral animal, turns toward its right 

 or left side. It does not, in such instances, utilize its radial 

 symmetry by starting to move backward or to the right angle 

 or to any other angle to its longitudinal axis. 



