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It is not certain that the lancet plates represent any of those plates 

 which in the Crinoidea are usually called radials." They are so arran- 

 ged that if they were loosened from the walls of the cup, and their 

 smaller extremities turned upward, whilst their bases or larger ends 

 retained their position, they would stand in a circle around the apex, as 

 do the arms of an ordinary Crinoid. Their bases would alternate with 

 the apices of the deltoid plates. They would form the outside of the 

 arms, whilst the grooves and pinnulixj would be inside. Each would bear, 

 on its outer or dorsal aspect, two elongated sacks, the two hydrospires 

 that belong to the ambulacrum. I believe that the small groove in the 

 ambulacrum of Pcntremites was occupied by the ovarian tube only. If 

 this be true, and if, also, the lancet plates represent the radial plates of the 

 arms of the Crinoids, then the arm of Pentremltes would have the respira- 

 tory portion of the ambulacral system on its dorsal, and the ovarian portion 

 on its ventral aspect. 



In the true Crinoids, both the respiratory and ovarian tubes are situated 

 in tl^g grooves in the ventral side of the arm.* In the Crinoids the pin- 

 n\i\2Q are attached to the radial joints of the arm. In Pentremltes they 

 are not connected with the lancet plate, but with the pore plates. In P. 

 pyriformis they appear tome to stand in sockets excavated in the suture 

 between the pore plates proper, and the supplementary pore plates. Mliller 

 compared them to th« series of azygos plates, which underlie that portion 

 of the ambulacrum of Pentacrinus that runs from the mouth to the base 

 of the arm. These resemble the lancet plates, in their being azygos and 

 not connected with pinnulog ; but then, on the other hand, they differ from 

 them in having, a portion at least, of the respiratory tubes on their ventral 

 aspect. Mr. Rofe says that, " in many species of Pentremltes, if not in 

 all, this lancet plate is in reahty a compound plate, formed of two conti- 

 guous plates, extending from the bottom of the sinus to the top, and, 

 then turning right and left round the summit openings, they pass down 

 the adjoining sinus to form half its lancet-plate, leaving at the apex of the 

 body a pentagonal aperture, supposed to be the mouth. In some 



* Thomas Say tvIio ^vas the first to recognize|the Blastoidea as a group distinct from the 

 Crinoidea, also supposed the function of the ambulacra to be respiratory. He says, " I 

 think it highly probable that the branchial apparatus communicated with the surrounding 

 fluid through the pores of the ambulacrae, by means of filamentous processes ; these may 

 also have performed ihe office of tentacula, in conveying food to the mouth, which was 

 perhaps, provided with an exsertile proboscis ; or may we not rather suppose that the 

 animal fed on the minute beings that abounded in the sea water, and that it obtained 

 them in the manner of the Ascidia, by taking them in with the water. The residuum of 

 digestion appears to have been rejected through the mouth." (Jour. Acad. N. S. Phil 

 vol. iv, p. 296, 1825). 



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