412 Mr. E. Billings 07i the Structure of 



have been exclusively ovarian in its function. The hydro- 

 spires have no connexion whatever with the aiins, and are, 

 moreover, all of them entirely separated from each other. If, 

 then, they represent the ambulacra! system of the recent Echi- 

 nodcrms, it is quite certain that that system was at first (or in 

 the undeveloped stage in Avhich 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 Avitli the arms to 

 this extent, that these latter lie back upon them. The arms 

 are provided with pinnulaB ; but it is not at all certain that the 

 pinnulaa were in any direct communication with the hydro- 

 spires. It is evident that in all the Cystidea (and in none 

 is it more obvious than in Caryocrinus) there was no con- 

 nexion between the hydrospires and the pinnulfe. The main 

 difference (so far as regards the evidence of the presence or 

 absence of such a connexion) between Caryocrinus and Ca- 

 daster 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 

 Cadaster has a fine ambulacral 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 Blas- 

 toidea, I have not shown it in the diagram, but only the five 

 pores, 2>' 



No one who compares a Cadaster with a Pentremites (the 

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

 hyckospires of the two genera are perfectly homologous organs. 

 If we grind off the test of a species of the latter genus, select- 

 ing one for the purpose which has broad petaloid ambulacra, 

 such as those of P. Schultzii, the structure exposed will be 

 that represented in the diagram fig. 3. In Fetitrenntes, as in 

 Cadaster, the five hydrospires are divided into ten equal parts 

 by the five rays, al,a2, &c. In Cadaster these ten parts re- 

 main entirely separate from each other; but in Pentremites 

 they are reunited in pairs, the two in each interradial space 

 being so connected at their inner angles that their internal 

 cavities open out to the exterior through a single orifice or 

 spiracle (s, figs. 3 &4). This is best shown in fig. 4, intended 

 to represent the structure of P. elUpticus (Sowerby), as de- 

 scribed by Mr. Rofe, Geol. Mag. vol. ii. p. 249. In this spe- 

 cies the hydrospires, instead of being formed of broad sacs 



