1881.] On the Locomotor System of Echinodermata. 



5 



a distinct connexion with nerve-cells. The cells consist of an oval 

 nucleus and of a layer of protoplasm, which is generally seen to pro- 

 ject in two, or sometimes in three, directions — the several processes 

 often uniting with similar processes from adjacent cells, so as to form 

 a fibro-cellular chain or network. 



In preparations from portions of Echini treated with both chloride 

 of gold and osmic acid, we have succeeded in tracing the plexus over 

 the surface of the shell between the spines and pedicellaria?, and from 

 the surface of the shell to the capsular muscles at the bases of the 

 spines ; we repeatedly observed delicate fibres passing beyond the 

 muscles to end apparently under the epithelium over the surface of 

 the spines. 



In the case of the pedicellaria the plexus on reaching the stem runs 

 along between the calcareous stem and the surface epithelium, 

 to reach and extend over and between the muscular and connective 

 tissue fibres between the stem and the bases of the mandibles. The 

 plexus, now in the form of exceedingly delicate fibres connecting 

 small bipolar cells, reaches the special muscles of the mandibles. 

 In several preparations delicate fibres appeared to extend towards 

 the sensitive epithelial pad situated on the inner surface of each man- 

 dible a short distance from the apex. Although this plexus is espe- 

 cially related to the muscular fibres, lying over and dipping in between 

 them, it is also related to the surface epithelium, and delicate fibres 

 often extend from it to end under or between the epithelial cells. 



II. — Physiology. 



1. Natural Movements. — The ordinary crawling movements of Astro- 

 pecten aurantiacus are peculiar, the ambulacral feet acting the part of 

 walking-poles and cilia combined. Brittle-stars progress by using two 

 opposite arms upon the floor of the tank, with a movement like swim- 

 ming ; at each stroke the animal advances with a sort of leap, and 

 can thus travel at the rate of six feet per minute. The ordinary pro- 

 gression of Echinus and Spatangus is assisted by the co-ordinated 

 action of the spines, and when placed upon a flat surface out of the 

 water the animal advances by means of its spines alone. In Echinus 

 the lantern and pedicellarise are also used to assist in locomotion. 



All the Echinodermata that we have observed are able, when placed 

 upon their dorsal sorfaces on the floor of a tank, to recover their 

 normal position on their oral surface. The common star-fish does so 

 by twisting the ends of two or more of its rays round, so as to bring 

 its terminal suckers into action upon the floor of the tank, and then,, 

 by a successive and similar action of the suckers further back in the 

 series, the whole ray is progressively twisted round, so that its ambu- 

 lacral surface is applied flat against the floor. The rays which perform 



