160 Prof. E. J. Chapman on the Structural Relations 



position of the mouth, as usually given, is erroneous. In several 

 species, as in A. parasiticus and A. Kaskaskiensis of Hall 

 (' Geology of Iowa/ vol. i. part 2. pi. 25), the centre or origin 

 of the rays is a simple disk or rounded tubercle — incontestably, 

 no mouth : and hence we may fairly assume that, in other species 

 also, the mouth must be situated elsewhere. The question then 

 arises as to the real nature of the pyramidal orifice. This is 

 usually looked upon either as an anal orifice or as an ovarian 

 aperture. Neither of these views is by any means certain, nor, 

 indeed, apparently susceptible of proof. To consider this orifice 

 as the mouth, however, appears a still less satisfactory conclusion. 

 In the Crinoids proper, the true position of the mouth is still, 

 strictly speaking, unknown. It is considered in some genera to be 

 in the centre of the " vault/' or upper surface, and in others to 

 occupy an excentric position, as between two of the arms, &c. 

 This latter view is unsustained by any proof, beyond the mere 

 occurrence of an orifice at the points in question. The excentric 

 orifice may or may not be the mouth. But if we omit these 

 forms from consideration, and turn to those types of Radiata in 

 which the position of the mouth is no longer doubtful, that organ, 

 it will be seen, is invariably situated in the centre of the body, 

 except in the Family of the Spatangida, the highest family or 

 natural group of the entire series. In the other families of the 

 Echinida, in the Asterida, Ophiurida, and other orders in which 

 the position of the mouth is truly known, the mouth is always 

 central. This is evidently its normal position in the radiated type 

 of structure, and one, consequently, that we should scarcely 

 expect to see departed from, except in the case of those forms 

 which stand at the higher limit of the series. Unless this view 

 be adopted, we must almost necessarily assume that in the 

 Radiata there are certain natural groups (not yet thoroughly 

 worked out) which are perfectly unconnected with each other, 

 and in which, respectively, the higher forms foreshadow an ad- 

 vanced type of structure, whilst the lower forms present the 

 normal type. The higher forms of a low group, however lowly 

 organized as to their entire structure, will be thus, in certain 

 respects, in advance of the lower forms of a higher group. 

 Whatever grounds there may be to believe that some law of this 

 kind really holds good in Nature, its application in the present 

 place would be evidently forced. Discarding, therefore, the idea 

 that in the pyramidal orifice of the Cystideans and Agelacrinites 

 the mouth is represented, this latter organ must be sought for 

 in another place. Reasons have already been stated against this 

 being the centre of the rays. Its true position will be found, I 

 believe, in the centre of the under side of the body. But, it 

 may be urged in objection to this, the genus Agelacrinites is 



