836 
MR. G. J. ROMANES AND PROFESSOR J. C. EWART 
the large branches already referred to as escaping with the inner row of pedicels is 
traced through the oral floor after sending a branch to the foot, it breaks up into 
delicate fibres, some of which run towards the bases of the adjacent spines and 
pedicellarise, while others run inwards a short distance towards the oral aperture. 
Either in connexion with, or anatomically independent of these filaments from the 
lateral branches of the nerve trunks, there is an external plexus lying almost imme¬ 
diately under the surface epithelium and extending from the shell to the spines and 
pedicellarise. The fibres (Plate 80, fig. 15) of this plexus closely resemble those of the 
lateral branches of the trunk; but generally they are smaller in size and have a 
distinct connexion with nerve cells. The cells consist of an oval nucleus and of a layer 
of protoplasm, which is generally seen to project 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 pedicellarise ; and from the surface of the shell to the capsular muscles 
at the bases of the spines (Plate SO, fig. 16). Further, we repeatedly observed delicate 
fibres passing beyond the muscles, apparently to end under the epithelium over the 
surface of the spines (Plate 80, fig. 13, T). 
In the case of the pedicellarke, the plexus on reaching the stem runs along between 
the calcareous axis and the surface epithelium, to reach and extend over and between 
the muscular and connective tissue-fibres between the calcareous axis and the bases 
of the mandibles (Plate 80, fig. 13, l", and fig. 18). 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 (Plate 80, fig. 13, s ) situated on the inner surface of each 
mandible, a short distance from the apex. Although this plexus is especially 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. 
PART II.—PHYSIOLOGY. 
§ I. Natural Movements. 
It is desirable to begin this account of the physiology of the locomotor system, with 
a somewhat full account of the natural movements exhibited by the various species of 
Echinoderms which we have had the opportunity of observing. This is desirable, not 
only because it is necessary to study the natural movements before we can be in a 
position to appreciate the results of the following experiments, but also because these 
natural movements form in themselves a study of considerable interest. 
1. Star-fishes.— (A) Taking the common Star-fish (Uraster rubens) as our start- 
