49 



the petal-like divisions, and one to the inner areas of each petal where it spreads, 

 insinuates between, and attaches itself to the marginal radii. These hitler five 

 portions of the muscular integument, effect the lateral motion of the column by 

 relaxing on one side and drawing in on the other, thus allowing to each joint a 

 slight motion tending to bend the column in a degree equal in quantity to the 

 thickness of the marginal radii, the other h've portions of the muscular integu- 

 ment, draw inwards the muscular space between the petal like divisions, whilst 

 a lateral contraction is effected from those investing the areas of the petals. 



The smooth depressed space within the petal-like divisions and between 

 them, may furnish us with a reason why in the pentacrinite, the contrivance of 

 a ball of fluid enclosed between two concave cups, as noticed in the column of 

 Apiocrinites rotundus is done away; since the muscle covering them being 

 less divided at its central junction, can exercise a superior force, and by offer- 

 ing a smooth and yielding surface to the joints, can more readily effect and 

 aid a lateral motion of the column. The concave spaces resulting from the de- 

 pressed areas of each petal, and thus included between the contiguous joints, 

 are also sufficiently analogous to those inttrvertebral cavities, and calculated 

 for the same application where circular motion is not required. 



Tn the late Mr. TOBIN'S specimens, the greater or less muscular state of 

 the column at different periods of growth is beautifully exhibited ; (Pi., i.) for 

 although near the fractured end, the joints (which, as has already been ob- 

 served, are partially consolidated, and tilled up by calcareous secretions) dis- 

 play very slight traces of muscular contraction between the petal-like divisions ; 

 yet higher up, this contraction becomes very conspicuous, the joints here become 

 thinner, and are alternately disposed, a smaller and thinner joint succeeding a 

 larger and thicker, to allow a greater freedom of motion, till near the apex this 

 change is so conspicuous that the small cues resemble thin leather-like inter- 

 positions, 



It can also, I think, be asserted from this appearance, with great certainty, 

 that it is here the column of the animal grew during the energy of its life, 

 and elongated in proportion to other parts. The thin and highly muscular in- 

 terposing joints, gradually became thicker, being favourably placed, from their 

 greater vicinity to the abdominal cavity, to receive a more copious supply 

 of nourishing fluids and calcareous matter through the alimentary canal, than 

 the joints at a greater distance. I also apprehend that from the great tnuscu- 



H 



