Revieios — Wachsnmth S^ Springer's Ilonograph on Crinoids. 327 



served the double purpose of transmitting nutrient fluid to the 

 mesoderm cells depositing the outer layers of stereom, as the stem 

 and root grew wider by concentric accretion, and of aerating the 

 same fluid by bringing it near the oxygenated sea-water. 



II 



III 



The Development of ' Pores ' from Cirri. 



I. — Crotalocrinns ; portion of Root, -with brancliiug cirri below, aud attachments of 

 cirri in upper part. These latter show the axial canal that passes from the 

 main axial canal of the stem, through the thickness of the columnals, to each 

 cirrus, and continues to the end of the cirrus. Specimen from Silurian of 

 Gotland. British Museum, regd. E 1,273. Nat. size. 



Cystocrinus tennesseensis ; part of stem, showing stumpy aborted cirri, with 

 axial canals opening at their ends. Niagara group, W". Tennessee. Brit. 

 Mus., regd. E 53 (a). Nat. size. 



-Crotalocrinus ; part of stem, showing crenulate sutures between columnals, 

 and on the columnals the atrophied attachments of cirri ; compare with the 

 extreme upper part of Fig. I. Wenlock Limestone, Much Wenlock. Brit. 

 Mus., regd. E 6,633. x 5 diam. 

 IV. — Crotalocriinis ; part of stem, showing total disappearance of cirrus-attachment, 

 and only the axial canals remaining as ' pores ' piercing the columnals. 

 Silmian, bed / of Liudstrom, near Pafvalds, Gotland. Brit. Mus., reo-d. 

 E 6,139. X 5 diam. ° 



It is not quite clear what importance Messrs. Wachsmuth and 

 Springer attach to the conception of the dorso-central. It is not 

 necessary to repeat the reasons why "the dorso-central, considered 

 as a morphological element of the Echinoderm type, has got to go." ^ 

 There remains, however, the question whether it can be considered 

 an independent element of the Crinoid skeleton. Since the distal 

 segment of the stem always remains distal, no fresh ossicles ever 

 being developed between it and the sea-floor, it is clear that it 

 is homologous in all crinoids. In Antedon, as we have long known, 

 this distal element forms no part of the adult, but is left behind 

 when the animal enters on its free-moving existence. This is also 

 the case in the Pentacriniute, and Messrs. Wachsmuth and Springer 

 have collected evidence to show that the same phenomenon was 



The text-book writer among the Echinoderms," oj). cit. antea. 



