DR. WILSON ON THE NERVOUS SYSTEM OF THE ASTERID^. 109 



the ambulacral groove, pi'esents near its upper part a quadrilateral tubercle (a), which 

 gives attachment to connective fibrous tissue. This tissue stretches across from one side 

 to the other, like a ligament, enclosing between itself and the vertical portion a trian- 

 gular space, which lodges the water-vessel (b) for the supply of the vesicles (j i). We may 

 term it the inferior ligament. 



Both in the superior and inferior ligaments, in addition to the connective tissue, fusiform 

 nucleated cells may be seen imbedded amongst flat riband-shaped fibres *. These structures 

 are undoubtedly contractile, performing the function of muscle ; for by the contraction 

 of the inferior ligament the animal can adduct its divergent plates, and thus diminish 

 the width of the ambulacral groove, and by a similar action of the superior ligament an 

 opposite result is produced. It is evidently for this purpose that the upper margins of 

 the vertebral plates are bevelled so as to recede from each other. And, though the supe- 

 rior muscle acts under great mechanical disadvantage (the fulcrum of the lever x d 

 being at a x), yet a very small amount of requisite contraction compensates for this 

 waste of power. 



The margin of the oblique portion of the plate, below the inferior ligament, gives attach- 

 ment to the muscular arrangement of the sucker ; and its surfaces have here shallow ver- 

 tical grooves on either side, for the passage of the cylindrical tubes from the vesicles. The 

 latter rest on the dorsal or csecal aspect of the vertebrae. To the inferior extremity of 

 each plate is articulated a large rounded calcareous mass \, bearing on it from 8 to 10 

 or more spines (//). Connective tissue, of the same nature as that forming the liga- 

 ments, binds, capsule-like, each spine to the rounded mass, and these to the vertebrae. 

 The spines are moveable, as well as the masses on which they rest, and serve to close in 

 the ambulacral groove. They, as also the external portion of the plates, are covered by 

 the integument, there being, however, little more than cuticle over the former. The 

 caecal aspect of the plates is invested with a peritoneal layer. The ambulacral arches thus 

 formed by the series of symmetrical plates do not rest against one another in parallel 

 vertical planes, but incline obliquely towards the centre of the body, so as to rest on its 

 proximal neighbour, and, in turn, be overlapped by its distal one. 



The ambulacral cord lies between the inferior ligament and the integument (</). When 

 examined without the use of reagents, it is found to consist of almost homogeneous but 

 finely granular nucleated cells, resting upon the integument, or pendent from the ligament, 

 along the mesial line. They lie in a coarsely granular and molecular matrix, and in a 

 transverse section always appear unipolar. 



Their filaments have the double contour iudicative of primitive sheath and central axis, 

 and are equally transparent with the cells. Some of them may be seen crossing the in- 

 ferior ligament, occupying the space between the contiguous arches, and passing ixpwards 

 round the water-vessel as far as the superior ligament, where they are generally lost sight 

 of. Occasionally I have detected fibres turning round the upper extremities of the plates ; 

 but it is difiicult to distinguish these from the fibres of the ligament. Many filaments run 

 obliquely through the substance of the inferior ligament, diverging from the cells towards 

 the vesicles. Other fibres are directed transversely outwards ; but I have not been able 



* Plate Xm. fig. 2. t lljiJ- %• 1- «• 



