1881.] On the Locomotor System of Echinodermata. 



7 



fully tenacious grasp of the forceps is timed as to its duration with 

 an apparent reference to the requirements of the pedicels, for after 

 lasting about two minutes (which is about the time required for the 

 suckers to bend over and fix themselves to the object held by the 

 pedicellariee, if such should be a suitable one), this wonderfully tena- 

 cious grasp is spontaneously released ; and, 3rd, the most excitable part 

 of the trident pedicellarise is the inner surface of the mandibles, about 

 a third of the way down their serrated edges — -i.e., the part which 

 a moving body cannot touch without being well within the grasp of 

 the forceps. When the forceps are closed, they may generally be 

 made immediately to expand by gently stroking the external surface 

 of their bases. 



With regard to stimulation of the spines, if severe irritation be 

 applied to any part of the external or internal surface of an Echinus, 

 the spines all over the animal take on an active bristling movement. 

 The tubercles at the bases of the spines are the most irritable points 

 on the external surface. 



With regard to stimulation of the pedicels, if an irritant be applied 

 to any part of a row, all the pedicels in that row retract in succession 

 from the seat of stimulation, but the influence does not extend to 

 other rows. A contrary elfect is produced by applying an irritant to 

 any part of the external nerve plexus, all the pedicels being then 

 stimulated into increased activity. Of these antagonistic influences, 

 the former, or inhibitory one, is the stronger, for if they are both in 

 operation at the same time the pedicels are retracted. 



Star-fish (with the exception of brittle-stars) and Echini crawl 

 towards, and remain in, the light; but when their eye-spots are 

 removed they no longer do so. When their eye- spots are left intact 

 they can distinguish light of very feeble intensity. 



3. Section. — Single rays detached from the organism crawl as 

 fast and in as determinate a direction as do entire animals. They 

 also crawl towards light, away from injuries, up perpendicular sur- 

 faces, and, when inverted, right themselves. Dividing the ray-nerve 

 in any part of its length has the effect of destroying all physio- 

 logical continuity between the pedicels on either side of the division. 

 Severing the nerve at the origin of each ray, or severing the nerve- 

 ring between each ray, has the effect of totally destroying all co- 

 ordination among the rays ; therefore, the animal can no longer crawl 

 away from injuries ; and, when inverted, it forms no definite plan for 

 righting itself. Each ray acting for itself, without reference to the 

 others, there is, as a result, a promiscuous distribution of spirals and 

 doublings, which, as often as not, are acting in antagonism to one 

 another. This division of the nerves usually induces, for some time 

 after the operation, more or less tetanic-like rigidity of the rays. 

 The operation, however, although so completely destroying physio- 



