1869.] BILLINGS — ON THE STRUCTURE OP CRINOIDEA, ETC. 429 



are arranged in five groups. Commencing at m v and going 

 round by 1,2, &c., there are four in the first group ; one in the 

 second ; four in the third; one in the fourth and four in the fifth. 

 These five groups represent the five ambulacral canals of the 

 recent echinoderms. In the specimen from which this diagram 

 was constructed there are the bases of fifteen free arms to be seen 

 situated at the outer extremities of the dotted lines. At the base 

 of each arm there is a small pore, p, which I believe to have been 

 exclusively ovarium in its function. The hydrospires have no 

 connection whatever with the arms and are, moreover, all of them 

 entirely separated from each other. If, then, they represent the 

 ambulacral system of the recent echinoderms, it is quite certain 

 that that system was at first, (or in the undeveloped stage in 

 which it existed in the Cystidea,) destitute of the oesophageal 

 ring. 



In Cadaster a further concentration of the respiratory organs is 

 exhibited. There are here only five hydrospires and they are 

 all confined to the circle around the apex. Two of them are 

 incomplete in order to make room for the large mouth and vent 

 (m V, fig 2.) They are each divided into two halves by an arm, 

 al, a2, &c. They are only connected with the arms to this extent, 

 that these latter lie back upon them. The arms are provided 

 with pinnulse, but it is not at all certain that they (the pinnulas) 

 were in any direct communication with the hydrospires. It is 

 evident that in all the Cystidea^ (and in none is it more obvious 

 than in Garyocrinus,') there was no connection between the 

 hydrospires and the pinnulse. The main difiference (so far as 

 regards the evidence of the presence or absence of such a connec- 

 tion) between Garyocrinus and Godasfer, consists in this, that in 

 the former the arms are erect and do not touch the hydrospires, 

 whereas in the latter they are recumbent and lie back upon them. 

 Each of the arms of Godaster has a fine ambuhicral groove and 

 all of the grooves terminate in a single central aperture. But as 

 this aperture was covered over by a thin plated integument, as in 

 the Blastoidea, I have not shown it in the diagram, but only the 

 five pores, p. 



No one who compares a Godaster with a Pentremites (the 

 internal structure of the latter being visible) can doubt that the 

 hydrospires of the two genera are perfectly homologous organs. 

 If we grind ofi" the test of a species of the latter genus, selecting 

 one for the purpose which has broad petaloid ambulacra such as 



