MOXOGRAPH OF THE EXISTING CRINOIDS. 



353 



proximity, mesenchyme cells become prominent, and the formation of a new 

 sacculus is begun. 



The first sacculi, five in number, appear in the Antedon larva shortly before 

 the breaking through of the vestibulum. 



Figs. 758-764. — 7ri8. Disk of PsNTAMETROCaiNOS vaeians. 759. Disk of Thaumato.\ietba bobealis. 

 760. Ventral tikw of foktion of pinnule of Pekissometra .selbne. showing co\-ering plates 

 closed down over ambulacral groove. 761. Lateral view of portion of dried pinnule of Neo- 



MBTBA SPINOSISSIMA, SHOWING COVERING PLATES CLOSED DOWN OVER AMBULACBAL GBOO\-B. 762. TOB 

 OP GBODSE (BONASA UMBBLLATA), SHOWING SCUTES DEVELOPED IN WINTER WHICH AUK SIMILAB TO 

 COVERING PLATES (DRAWING BT AUTHOE). 763. VENTRAL VIEW OF PORTION OF DRIED PINNULE OF 

 COMATILIA IRIDOMETEIFORMIS, SHOWING COVERING PLATES (DRAWING BT AUTHOE). 764. LATERAL VIEW 

 OF DRIED PINNULE OF COMATILIA lEIDOMETRIFORUIS, SHOWING COVERING PLATES (DRAWING BT AUTHOE). 



In describing the formation of a sacculus Keichensperger says that a limited 

 number of mesenchyme cells assemble, usually forming a small spherical cell mass. 

 These cells possess a circular nucleus with prominent nuclear bodies. The small 



