BILATERALITY. 129 



of the living Crinoids, it has been agreed among naturalists 

 that the small aperture, which is in the slight prominence (a) 

 near the mouth, is the posterior terminus of the digestive canal. 

 Taking, now. the two opposite extremities of this canal as the 

 extreme points of a line, we may project that line, as a plane, 

 through the body toward its stem, so as to divide the Crinoid 

 into right and left portions. There is not anything about the 

 animal which militates against this method of topography ; and 

 at the same time I would say that there is not the least rudi- 

 ment of radiation in the disposition of the components of this 

 body. There are traces, that is, scars on each side of the pro- 

 boscis (i), where it is thought that arms were attached ; but 

 these are as definitely arranged in regard to right and left as are 

 the arms of a cuttle-fish or squid (chap. xi. fig. 124). There 

 are other kinds of Crinoids whose arms are much more conspic- 

 uous for their right and left arrangement than those of Hemi- 

 cosmites.* 



What then do you suppose would have been the decision of 

 a naturalist had he lived at that time, far down at the bottom 

 of the Silurian period, at the period of the first appearance of 

 life upon the earth ? He certainly would never have thought of 

 such a thing as a radiate type ; simply because there is nothing 

 in these animals to suggest such an idea. I might also refer you 

 to the embryonic or earliest stages in the growth of Zoophytes, 

 and you would see there also that the radiate character is either 

 very feebly represented, or altogether absent, w^hilst the bilateral 

 feature stands out prominent ; but I cannot at present go into 

 many details, as that would anticipate what you will hereafter 



aperture of the reproductive organ ; p, the plates of the shell ; s, the point of 

 junction of the shell with the stem. From Pictet. 



* For the benefit of those who may object that the ovarian opening (fig. 66, 

 0) of Hemicosmites is unsymmetrically placed, and therefore is out of relation 

 with the bilateral plane, I would propose, as an answer, to show on the same 

 score, that the worm Bonellia (chap. xri. fig 126) is not a bilateral animal, inas- 

 much as the aperture of its reproductive organ is a little on one side of the me- 

 dian line ; or that a man is not a bilateral creature, because he uses his right 

 hand more than his left. 

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