278 THE CANADIAN NATURALIST. [Sept. 



centre ot the radial system, as it does in all the existing Echino- 

 dermata. On this point Prof. Wyville Thompson has the follow- 

 ing observations : 



" I can see no probability whatever in the opinion lately advo- 

 cated by Mr. Billings, and which has received some vague support 

 from the writings of De Koninck, and others, that the 'pyramid ' 

 in the Cystideans is the mouth, and that the aperture whence the 

 ambulacra radiate is simply an ' ambulacral orifice.' Such an 

 idea appears to me to be contrary to every analogy in the class. 

 There can be no doubt of the existence of distinct openings for 

 the passage of the ambulacral nerves and vessels from the calyx 

 of many of the paleozoic crinoids ; but I think we must certainly 

 assume that in this, as in all other known instances, these vessels 

 had their origin in an annular vessel surrounding the mouth. In 

 the whole class the sesophageal circular canal seems to be the 

 origin and centre of the ambulacral system. It is the first part 

 which makes its appearance in the embryo, and is fo permanent 

 and universal that one could scarcely imagine a radiating ambul- 

 acral vessel rising from any other source. The early origin of 

 this important vascular centre, in this annular form and in this 

 position, evidently depends upon, and is closely connected with, 

 the origin of the nervous system in the sesophageal nerve ring, 

 constant in the whole invertebrate series."'* 



With all due deference I cannot admit that we must assume 

 that, in the Cystidea, the ambulacral tubes had their origin in 

 " an annular vessel surrounding the mouth." It is true that such 

 a vessel does surround the mouth of existing Echinodermata, but 

 there is no essential or direct physiological connection between the 

 two organs. Their functions are exercised independently of each 

 other. There is no organ issuing out of the alimentary canal that 

 communicates with the annular vessel. This latter might be situ- 

 ated in any other part of the body and still perform its functions, 

 provided there were a connection between it and the ambulacra. 

 In this class, the position of the various organs, in relation to 

 each other, and also to the general mass of the body, is subject to 

 very great fluctuations. Thus, the mouth and vent are separated 

 in some of the groups, but united in others, while either, or both, 

 may open out to the surface directly upward, or downward, or at 

 any lateral point. The ovaries may be either dorsal or ventral, 



* Edinburgh N. Phil. Jour., vol. xiii., p. 112, 1861. 



