DIRECTION OF LOCOMOTION OF THE STARFISH 27 



ward from the dorsal position until it comes to lie on the pos- 

 terior border of the periphery, while conversely, the inouth may 

 be located considerably forward of the mid-ventral position. 

 Now it is of considerable interest to note that the region which 

 is anterior is an ambulacrum, corresponding to a ray of the 

 starfish, and that the 7nadreporite always lies in the inter-ambu- 

 lacral area next to the right of the anterior ambulacrum. If, there- 

 fore, we designate the different areas with relation to the madre- 

 porite as we have done in the starfish, it will be observed that 

 the anterior ambulacrum is e {IIIY^ and this is the ray ivhich icas 

 found to function as the 'physiological anterior' in Asterias. These 

 radii in the two cases are accordingly clearly analogous physio- 

 logically, but whether they are morphologically homologous, it 

 would be hazardous to state, since the complications of embryo- 

 logical development make this point practically impossible 

 to determine. The physiological relationship may be readily 

 understood by comparing figure 9, which is a diagrammatic 

 representation of a spatangoid, with figure 5 (p. 11) which 

 similarly represents the starfish. 



In holothurians the morphological and axial relations are so 

 different from those of the forms we have been discussing that 

 there would be little of value in a comparison. It is of interest 

 to note, however, that Pearse ('08, p. 269) found in Thyone 

 a preferential use of certain tentacles. As to locomotion 

 however, he states (p. 264) that 'the animal may move in any 

 direction,' and (p. 266) that 



Individuals move with the posterior end in advance as often as with 

 the anterior end, and although the long axis of the body is as a rule 

 approximately parallel with the direction of locomotion, animals often 

 move a long distance (as much as 12 cm.) with the body at right angles 

 to the direction of movement, that is, they move straight toward the 

 right or left. 



" The anus consequently lies in the inter-ambulacral area (5) corresponding 

 to the inter-radius be of the starfish. In certain of the radial sea-urchins (e. g. 

 Strongylocentrotus) the anus occupies an eccentric position in the periproct, 

 lying nearer to the border opposite interambulacrum 5 {be). A line drawn through 

 ambulacrum in (e) and interambulacrum 5 (be) therefore marks the beginning 

 of a bilateral symmetry and presages the condition found in the spatangoids. 



