Sensory reactions of Holothuria surinamensis. 241 



(0,6 cm long), on the under sides of large stones in situations whei-e 

 there was often considerable wave action. 



On a solid surface Holothuria progresses by the combined action 

 of pedicels, buccal tentacles, and a peristaltic activity of the body 

 musculature (cf. Hékouaed, 1889, p. 539). The last factor is not 

 always evident, and the first one is relativel}' unimportant, except 

 in the larva (Edwaeds, 1909, p. 217). My observations on the rôle 

 of the pedicels agree with those of Pearse (1908, p. 265), in that 

 in neither Thyone nor Holothuria is there anj^ pushing action of the 

 pedicels such as Jennings (1907, p. 99) described in the horizontal 

 locomotion of Asterias forreri. The body is never lifted clear of the 

 substratum. The tube feet become much more readily attached to 

 a rough surface, such as that presented by a stone, than they do 

 to the smooth surface of a glass plate. Progression motions always 

 begin at the anterior end, with the tentacles extended; the most 

 anterior pedicels contract first, and a sort of wave of contraction 

 passes down the array of tube feet. In H. captiva there is some 

 lack of coordination observable in this process, as the most posterior 

 pedicels are frequently much stretched, and pulled loose, by the 

 forward extension of the anterior part of the body. The rate of 

 locomotion on a horizontal smooth surface is of the order of magni- 

 tude of one centimeter in four minutes, in the absence of directive 

 stimuli. Under certain conditions the body may, in crawling, be- 

 come twisted into a more or less complete spiral on the long axis ; 

 such twisting begins at the anterior end, which in this process is 

 enormously elongated as compared with its ordinary appearance. 

 Neither is the animal rigidly straight, for it may become bent at 

 any level, and even completelj^ doubled on itself. Progressive waves 

 of local contraction and relaxation of the body musculature have been 

 mentioned as 'peristaltic' waves ; such waves were observed to begin at 

 any level, and to move anteriorly or posteriorly with about equal fre- 

 quencies. They are rather slow, the most rapid ones traveling at 

 a rate of one centimeter in four to ten seconds, and their principle 

 function in locomotion is the extension of the anterior end of the 

 animal. 



Cole's (1913a) statistical study of the movements of the starfish 

 Asterias forbesi in the absence of directive stimuli has brought out 

 the fact that the part of the animal most frequently in advance is 

 the region in closest proximity to the madreporite. This ''physio- 

 logical anterior', as Cole points out, corresponds to the morpho- 



