62 THE APODOUS HOLOTHURIANS 



usually fully extended, and while the tentacles clearly assist, the principal 

 means of locomotion is by the alternate contraction and extension of the body- 

 wall, due to muscular movements. The contraction of the longitudinal muscles 

 draws the posterior end of the body forward, and their relaxation, accom- 

 panied by the contraction of the circular muscles, forces the anterior end 

 ahead; for since the body-cavity is filled with fluid, the decrease in its di- 

 ameter, due to the contraction of the circular muscles, necessarily results in its 

 increased length, and this causes a forward movement, since the body is pre- 

 vented from slipping backward by the marked projection of the calcareous 

 particles. This projection is brought about by the contraction of the circular 

 muscles — a contraction that begins at the posterior end of the body and moves 

 forward to the head. In species which are distinctly subterranean the calca- 

 reous particles of the rear of the body are frequently much larger and more 

 numerous than those anteriorly, in accordance with their use. When moving 

 through sand or mud, the tentacles play an important part in the progress by 

 separating and loosening the particles and pressing them apart, thus permit- 

 ting the anterior end to be forced forward by the contractions of the body 

 muscles. The subterranean species often move about in burrows, gathering 

 food as they go, the glands of the skin providing a secretion which serves to 

 give the walls of the burrows a certain smoothness, as well as increasing their 

 firmness. Synaptas can turn in their burrows, although they do not usually do 

 so, and backward movements are possible, but not frequent. In collecting food, 

 Synaptids do not lie in their burrows, with their tentacles extended on the 

 surface of the sand, waiting for what may be brought them, but they gather 

 it as they creep about, picking it up by the tips of the tentacles and passing it 

 inward to the mouth. The movements of the tentacles and digits are effected 

 partly by the contained fluid of the water-vasciilar canals, but mainly by the 

 longitudinal muscles lying on the inner surface. Under pressure from behind, 

 due to muscular contraction of polian vesels and perhaps of the water-ring it- 

 self, the fluid tends to straighten the tentacles, while the contraction of their 

 longitudinal muscles tends to bend them inward. Movements to avoid enemies 

 are occasionally like those already described, but may take the form of sud- 

 den, active, and very powerful muscular contraction. Similar movements may 

 be produced artificially by changes in environment or by unnatural stimuli. 

 The results of these movements depend of course upon the muscular systems 

 involved. ^Vlieu the whole animal is affected, the body becomes reduced in 

 bulk, both the longitudinal and lateral diameters being evidently decreased; 

 the tentacles may be so strongly contracted as to be entirely invisible, crowded 

 as they are into the now concave surface of the oral disc. Contraction of 

 the circular muscles is always more max"ked than of the longitudinal, and may 

 go so far as to break the body up into a number of fragments. Such disas- 



