STRUCTURE OF THE CYSTIDEiE. 19 



mine accurately the functions of the several orifices that have 

 occasioned so much discussion among the eminent naturalists and 

 palaeontologists who have written upon the structure and affinities 

 of these fossils. It is not however probable that a single specimen 

 retaining even a vestige of the soft parts will ever be discovered, 

 and we are compelled therefore to content ourselves with the next 

 best method of conducting the investigation. The only course is 

 to proceed by examining and comparing the offices of the ambulacral 

 grooves and apertures of some of the existing species of Echinoder- 

 mata wliich have been dissected by observers of good authority. 

 In pursuance of this plan, I shall here notice briefly such points in 

 the organization of the Star-fishes and recent Crinoids, as appear to 

 have a direct bearing upon the subject. 



I. Ambulacra of the Starfishes. 



The Star-fishes are not closely related to either the Crinoideae or 

 the Cystideae in the structure of their skeletons, but they present 

 the most perfect examples of Echinoderms with all the ambulacral 

 vessels located in grooves upon the outside. The mouth is situated 

 in the centre of the under-side of the body, and the ambulacra 

 consist of a set of deep furrows which radiate therefrom to the outer 

 extremities of the rays. They contain the following organs, all of 

 which communicate with the interior by passing inward through 

 the mouth. 



1st. The aquiferous canals^ consisting of a set of long tubular 

 vessels extending the whole length of the furrows and entering the 

 mouth. They are attached to, or originate in, another vessel, which 

 forms a ring around the mouth, inside of the body. Connected with 

 these vessels, and situated in the grooves, are two or four rows of 

 suctorial feet, the whole constituting the most extraordinary system 

 of locomotion known. 



2d. The nerves of the ambulacra. These are also connected with a 

 ring around the oesophagus, and pass out from the interior through 

 the mouth. The main trunks lie along the bottoms of the ambula- 

 cral grooves, and send out branches to the suctorial feet. 



3d. The blood-vessels, which also proceed from circular canals in 

 the interior, and reach the ambulacra through the mouth. 



The aquiferous canals and suctorial feet, with their nerves and 

 blood-vessels, constitute the ambulacral system of the Star-fishes ; 

 and as all the organs are situated on the outside of the animal, and 



