396 Prof. J. Reid on the Anatomy and 



the hinge-process is not unfrequently found in the position into 

 which it is brought by the action of the elevator muscle. These 

 muscular fibres present no transverse strise, can contract and relax 

 with rapidity, and become shorter and thicker during their con- 

 traction. The movements of the body upon the polypidom are 

 effected by the pedicle, and are as follows: — Suppose it to be 

 attached to one of the edges of the polypidom, and the concave 

 or upper surface to be looking upwards in the line of the long 

 axes of the cells, it can turn slowly outwards over the edge of 

 the polypidom until its concave surface looks directly outerward, 

 and it then returns to its former place : it may also turn in- 

 wards until the concave surface looks across the cells. This 

 movement being suspended, it exhibits at intervals a nodding 

 motion, the concave and convex sm'faces being alternately de- 

 pressed towards the anterior surface of the polypidom. When 

 the concave surface is carried downwards, the hinge-process is 

 slightly separated from the body ; but when the convex surface 

 is depressed, it is again approximated. These last movements 

 of the hinge-process are probably in a great measure mechanical, 

 and occasioned by it rubbing over the surface of the polypidom 

 during the downward motion of the concave surface. The pedi- 

 cle consists of two parts : a posterior and dense portion which is 

 attached to the internal surface of the inferior edge of the pro- 

 cess of the polypidom to which it is fixed, and passes inwards 

 through the inferior aperture in the concave surface of the body 

 to be inserted into the lower part of the internal surface of the 

 convex surface of the body; and an anterior portion, more trans- 

 lucent and less dense, which is prolonged downwards into the 

 process, and forwards to the middle aperture in the concave sur- 

 face of the body and the attached end of the hinge-process. In 

 the nodding movements when the convex surface is moved down- 

 wards, the posterior edge of the pedicle contracts and becomes 

 bent so as to form an acute angle; and it relaxes while the con- 

 cave surface of the body is moved downwards, resembling the 

 contractile movements of the stalks in Pedicellina echinata. I 

 have never had an opportunity of observing the changes in the 

 pedicle during the other movements of the body under a high 

 magnifying power, as this can only be done under certain condi- 

 tions not easily to be obtained. The anterior portion of the 

 pedicle has more of the appearance of a membranous than a con- 

 tractile structure, and contains several small nucleated cells. A 

 similar structure is found in the upper chamber of the body, 

 and is prolonged through the upper aperture in the concave sur- 

 face into the hinge-process. I have not been successful in ob- 

 serving contractile movements in this structure, if it really pos- 

 sesses this function, and I believe that it is more connected with the 



