NO. 19.] ECHINODERMS OF CONNECTICUT. I2g 



horizontal plane. They can and do turn in their burrows, but 

 as a rule they make new tubes when they come to the surface. 

 They are seldom still, and the old idea that they remained in the 

 tube they have formed with their tentacles just above the surface 

 is scarcely true. Sometimes they assume that position, but sel- 

 dom remain so very long. They rarely leave their burrows and 

 come out on the surface of the sand, and I doubt if they ever do 

 so under normal conditions. 



" Passage through the sand is chiefly accomplished by means 

 of contractions and extensions of the body, but is materially 

 assisted by the tentacles. With the latter, which are almost con- 

 tinually in motion, the sand is loosened and the grains more or 

 less separated. By the contraction of the longitudinal muscles 

 the rear of the body is brought up nearer to the head, and then 

 the circular muscles contract and extend the body again. It is 

 prevented from slipping back by the anchors, which are elevated 

 by the contraction of the circular muscles and hold against the 

 sand. Since the contraction begins next to the rear end and 

 moves forward, the head end is pushed onward, the anchors 

 there lying flat in the skin. 



" This process of alternate contractions of the two sets of 

 muscles is very obvious to an observer, and takes place very 

 continuously, though not rapidly. In this way a Synapta can 

 move through the sand from 2 to 3 centimeters a minute, and an 

 inhcerens of average size can get entirely out of sight in 5 or 6 

 minutes. One of the most remarkable provisions for the use 

 of the anchors in locomotion is their much greater abundance and 

 their considerably greater length in the posterior part of the body. 

 The use of this is clear when one realizes how the rear of the 

 body acts as the resisting base against which the muscles work in 

 pushing the anterior end forward." 



AUTOTOMY 



Both Thyone and Synapta often exhibit the habit of self- 

 mutilation or autotomy when subjected to very unfavorable con- 

 ditions or strong stimuli. Such reactions, however, occur in 

 very different ways in the two genera. Merely placing a Synapta 

 in a jar of clean sea water without sufficient sand to enable it 



