the Crinoidea, Cystidea, and Blastoidea. 



149 



supposes to be both mouth and vent, wliich accords Avith my 

 view (Mon. der Blastoideen, p. 378). In 1868 I discovered 

 the five small pores at the apical extremities of the ambulacral 

 grooves (Silliman's Amer. Jom-n. ser. 2. No. 97, p. 353, and 

 Ann. Nat. Hist. ser. 4. vol. iv. p. 76). In general it is diffi- 

 cult to see these pores ; but if a silicified specimen, which has 

 been fossilized in a calcareous matrix, be placed in an acid for 

 two or three minutes, the acid cleans them out, and they then 

 become distinctly visible. I believe these to be the pores 

 through which the ovarian tubes passed outward along the 

 grooves to the pinnulse. There are thus sixteen apertures in 

 the apex of Nucleocrinus — ten spiracles, five ovarian orifices, 

 and one oro-anal aperture. There are no true radial plates. 

 The whole of the test, with the exception, perhaps, of the 

 ambulacra, belongs to the perisomatic system. 



8. On the occurrence of Embryonic Forms among the Palceozoic 

 Echinoderms. 



Fi-. 8. 



Fij?. 9. 



Fio-. 10. 



Fijr. 11. 



Fig. 8. Bqnnnaria mtcrigera, Sars (copied from Miil- 

 ler) : a, the stomach ; b, part of the body of the 

 larva ; c, ambulacral centre, position of the per- 

 manent mouth, in this stage not open ; d, one of 

 the five ambulacral canals ; e, sand-canal ; /, ma- 

 dreporic plate ; in, entrance into the stomach ; 

 o, oisophagus ; p, larval mouth or pseudostome ; 

 r, oesophageal ring ; r, vent. Fig. 9. Ideal figure 

 described below. Fig. 10. Codonites stdliformis, 

 oblique view, to show both body and summit. 

 Fig. 11. Summit of fig. 10. 



No proposition in natural history has been more clearly 

 demonstrated than this — that in general the paleozoic ani- 

 mals resemble, both in external form and internal structure, 

 the embryonic stages of those of the same class at present 

 existing. Prof. Agassiz has long taught, in his lectiu-es and 

 various pubHcations, that this is especially observable in the 

 Echinodermata. Judging from the figures and descriptions of 



