VII. Morphology and Bionomics. 



1915 261 



Dr. Foerste applies this probable position of the living animal to 

 account for the reversed, i.e. solar, curve of the right posterior ray 

 (V). Speaking of an Agelacrinid with its fixed border of small 

 plates, he writes (p. 404) : " the position . . . would tend to increase 

 the tension along the upper part of the margin and just within the 

 adjacent part of the peripheral ring. If, at the same time, the anus 

 were dragged slightly downward and toward the left, the proximal 

 part of the right ray (No. 4) being directed up the supporting slope, 

 then the greatest tension would be on the upper, right-hand side of 

 the inner curve of the peripheral ring, possibly sufficiently below the 

 distal part of the right posterior ray (No. 5), in young specimens, to 

 loosen the contact between this part of the peripheral ring and the 

 immediately adjacent part of the posterior or anal interambulacral area, 

 and thus to admit of the curvature of the right posterior ray in a solar, 

 rather than a contrasolar direction." He finds support for this view in 

 the frequent presence of small plates "along the upper margin of the 

 anal pyramid", the animal being oriented as above described. However 

 that may be, the presence of such small plates on the solar side of the 

 anus (as already recorded in Edrioaster bigsbyi, 1914, PI. X, Fig. 9), 

 certainly indicates the presence of the underlying rectum, and thus 

 confirms the view that the gut had a solar coil. 



Dr. Foerste's argument has been given in his own words because it 

 is not easy to follow. If, however, he is correct as to the normal 

 position of the living animal, then we might find herein a very simple 

 explanation not merely of the solar curve of ray V, but of the 

 contrasolar curve of all the other rays. If the animal be laid on 

 a slope with the anus to the right of the mouth, then its various 

 parts are all subject to a gravitational force pulling down the slope 

 (text-fig. 1, b). The grooves, with their cover-plates, their heavy 

 floor-plates, and the various systems of organs associated therewith, 

 are heavier, i.e. subject to a stronger pull, than are the intervening 

 thinly plated areas. Consequently, when the grooves have grown out 

 so far that they have to turn one way or the other, they will naturally 

 turn in the direction of this pull. In other words, grooves IV, III, 

 and II will acquire a contrasolar bend, and groove V a solar bend. 

 Groove I should, on this hypothesis, most naturally have a solar 

 bend ; but owing to the greater width of the posterior interradius and 

 the primitive triradiate plan of branching, the proximal position of 

 this groove may very well have been directed almost straight down 

 the slope, so that the action of gravity would be indifferent, and its 

 course might rather be determined by the encroachment of groove II. 

 The strong re-curvature of groove V is further aided by gravity acting 

 on the rectum ; but in forms that had structures such as ampullae 

 on the inner side of the floor-plates, the posterior mesentery would 

 have opposed an effective bar to the passage of the groove across it. 



This hypothesis is suggested by facts observed in undoubtedly 

 sessile forms, and if it is to be applied to the Edrioasteridae, we must 

 suppose that they also assumed a similar sloping habit. Here, as in so 

 many similar cases, the field collector and observers have not supplied 

 the laboratory worker with the desired evidence. The section across 

 a specimen of Edrioaster big shy i (1914, Plate X, Fig. 10) shows that, 



