91 



Mr. Billings, and which has received some vague support from the writing? 

 of De Koninck and others, that the * pyramid ' in the Cjstideans 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 ves- 

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

 whole class the aesophageal circular canal seems to be the origin and cen- 

 tre of the ambulacral system. It is the first part which makes its appear- 

 ance in the embryo, and is so permanent and universal that one could 

 scarcely imagine a radiating ambulacral vessel rising from any other 

 source. The early origin of this important vascular centre, in this annu- 

 lar form and in this position, evidently depends upon, and is closely con- 

 nected with, the origin of the nervous system in the gesophageal 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 phy- 

 siological connection between the two organs. Their functions are exer- 

 cised independently of each other. There is no organ issuing out of the- 

 alimentary canal that communicates with the annular vessel. This latter 

 might be situated in any other part of the body and still perform its 

 functions, provided there were a connection between it and the ambulacral 

 ring. 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, internal or external, and asso- 

 ciated with either the mouth, the anus or with neither. The ambulacral 

 skeleton may be imbedded into and form a portion of the general covering 

 of the body or lie upon the surface, or borne upon free moving arms. In 

 genera belonging to the same family these relations are constant, or 

 nearly so, but are found to be extremely variable when different orders or 

 when remotely allied families are compared. 



While preparing my Decade No. 3, I investigated this subject anl 



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



