70 E. Billings on the structure of the Crinoidea, ete. 
and vessels from the calyx of many of the paleozoic crinoids ; 
ut I think we must certainly assume that in this, as in all 
other known instances, these vessels had their origin in an annu- 
lar vessel surrounding the mouth. In the whole class the xso- 
hageal circular canal seems to be the origin and center of’ the 
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 connec- 
tion 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 ves- 
sel. 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 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 fluctu- 
ations. 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, in- 
ternal or external, and associated with either the mouth, the 
anus or with neither. e ambulacral skeleton may be im- 
bedded 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 con- 
stant, 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 sub- 
ject and satisfied myself that in, at least, a large proportion of 
the paleozoic Crinoids, the mouth was disconnected altogether 
from the radial system. A great many species might be re- 
ferred to in which we can see both the center, from which the 
ambulacra proceed, and the mouth; and at the same time see 
that they are not in the same place. A long train of reason- 
ong 
ing is not necessary—only simple inspectio . It will be 
* Edinburgh N. Phil. Jour, vol. xii, p. 112, 1861. 
