1881.] On the Locomotor System of Echinodermata. 



9 



ment shows that these, if present, are distributed over all the internal 

 surface of the shell. For if the internal surface be painted with acid, 

 or scoured out with emery paper and brick-dust, the spines and 

 pedicellarias, after a short period of increased activity or bristling, 

 become perfectly quiescent, lie flat, and lose both their spontaneity 

 and irritability. After a few hours, however, the spontaneity and 

 irritability of the spines return, though in a feeble degree, and also 

 those of the pedicellarias in a more marked degree. These effects 

 take place over the whole external surface of the shell, if the whole 

 of the internal surface be painted with acid or scoured with brick- 

 dust ; but if any part of the external surface be left unpainted or 

 unscoured, the corresponding part of the external surface remains 

 uninjured. From these experiments we conclude : — 1st, that the 

 general co-ordination of the spines is wholly dependent on the 

 integrity of the hypothetical internal plexus ; 2nd, that the hypo- 

 thetical internal plexus is everywhere in intimate connexion with the 

 external, apparently through the calcareous substance of the shell ; 

 and 3rd, that complete destruction of the former, while profoundly 

 influencing through shock the functions of the latter, nevertheless 

 does not wholly destroy them. 



'Echini may be divided into pieces, and the pedicels, spines, and 

 pedicellarise upon these pieces will continue to exhibit their functions 

 of local reflex irritability, however small the pieces may be. If an 

 entire double row of pedicels be divided out as a segment, and then 

 placed upon its aboral end, it may rear itself up on its oral end by 

 the successive action of its pedicels, and then proceed to crawl about 

 the floor of the tank. We have therefore to meet the question : Is 

 the action of the ambulacral feet in executing these righting move- 

 ments of a merely serial kind — a, b, and c, first securing their hold on 

 the tank floor, owing to the stimulus supplied by contact, and then by 

 their traction tilting over the globe till d, e, and / are able to touch 

 the floor, and so on ; or does the righting action depend upon nervous 

 co-ordination ? We conclude that both principles are combined, the 

 action of the pedicels being serial, but also assisted by nervous co- 

 ordination. This conclusion is sustained by the experiment of shaving 

 off the spines and pedicels over one-half of one hemisphere — i.e., the 

 half from the equator to the oral pole. When then inverted and forced 

 to use their mutilated pedicel-rows, the Echini reared themselves upon 

 their equators, and then, having no more pedicels wherewith to con- 

 tinue the manoeuvre, came to rest. This rest was permanent, the 

 animal remaining, if accidents were excluded, upon its equator till it 

 died. The question, then, here seems to resolve itself simply into this : 

 Is the mechanism of the pedicels so constructed as to ensure that 

 their serial action shall always take place in the same direction ? For if 

 it can be shown that their serial action may take place indifferently in 



