252 Mr. E. Billings on the Structure of 



cral aperture. According to these views, tlie moutli of a Cys- 

 tidean does not stand in the centre of the radial system, as it 

 does in all the existing Echinodermata. On this point Prof. 

 Wy ville Thomson has the following observations : — 



" I can see no probability whatever in the opinion lately 

 advocated 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 aper- 

 ture whence the ambulacra radiate is simply an ' ambulacial 

 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 palaeozoic 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 oeso])hageal 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 so permanent and 

 universal that one could scarcely imagine a radiating ambula- 

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

 this important vascular centre, in this annular form and in 

 this position, evidently depends iipon, and is closely connected 

 with, the origin of the nervous system in the oesophageal 

 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 Echinoder- 

 mata ; but there is no essential or direct physiological connex- 

 ion 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 situated in any other part of the 

 body, and still perform its functions, provided there were a 

 connexion 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, 

 internal or external, and associated with either the mouth, or 

 the anus, or with neither. The ambulacral skeleton may be 



* Ediuburgh New Pliil. Journal, vol. xiii. p. 112 (18G1). 



