22 STRUCTURE OF THE CYSTIDEJE. 



All palaeontologists are agreed that this single aperture, found 

 in all the ancient Crinoideas, is both the mouth and the anus. 

 It is sometimes situated in the centre, equidistant from the bases 

 of the arms ; sometimes between the centre and the maroin : and 

 in a few species, such as in Canjocrhms ornatiis (Say), it is placed 

 at the edge of the cup, between the bases of two of the arms. 

 It is often level with the surface, but in many species it is in 

 the top of a long tube, the so-called proboscis, which is frequently 

 longer than the aims, and projects above their extremities when 

 they are erect. The ambulacral grooves however are only found 

 in the arms. They are not continued along the surface of the 

 body to the mouth, as in the Star-fishes and recent Crinoids; and 

 unless, therefore, there be some other provision made for their 

 entrance, it is difficult to see how the ambulacral canals, nerves 

 and blood-vessels could communicate with the interior. In those 

 species with the mouth not elevated they might find their way along 

 the surface, but it is improbable that they could do so without 

 leaving some trace of their passage ; and in Crinoids with a 

 proboscis it appears impossible that this course could be followed 

 at all. Provided therefore the usual description of the palaeozoic 

 Crinoids be correct, i. e., that they have only one aperture, then 

 their ambulacral system must have been totally disconnected from 

 the interior of the animal — a supposition that would be contrary to 

 all the analogies furnished by the structure of the other groups of 

 the Echinodermata. 



I have long been of opinion, that at the bases of the arms of the 

 extinct species there were special apertures provided for the passage 

 of the ambulacral vessels, but the evidence in my possession did not 

 appear sufficient of itself to warrant the publication of such a view. 

 Having had however, within the last few months, opportunities of 

 studying a large number of specimens in the collections of England 

 and France, I am now satisfied that there can be no doubt about 

 the matter. It is quite certain, that a great many of the extinct 

 Crinoids had either five, ten, twenty, or more of these openings, 

 and that through them the vessels of the ambulacra passed from 

 the grooves of the arms directly into the visceral cavity. While 

 examining the magnificent collection of the Geological Survey of 

 the United Kingdom I found many species which exhibit these aper- 

 tures in a most perfect state of preservation ; and upon consulting 

 Mr. Thomas A. Huxley, F.R.S., who is profoundly acquainted with 

 the details of the structure of all the orders of the Echinodermata, 



