MONOGRAPH OF THE EXISTING CRINOIDS. 355 



(4) Calcareous glands (Wyville Thomson). 



(5) Sense organs (W. B. Carpenter). 



(6) Symbiotic algae; Zooxanthellae (Vogt and Yung). 



(7) Slime glands (Bury). 



(8) Glands producing an amorphous secretion (Hamann). 



(9) Stores of reserve material (Cuenot). 



(10) Cement glands (Seeliger). 



(11) Defence organs (Reichensperger in his earlier work). 

 At the present time they are interpreted as excretory organs. 



TENTACLE PAPILL.E. 



The tentacle papillae consist of four or five elongated epithelial cells, with the 

 nuclei in the basal portion, each terminating in a fine sense hair. 



At the base of each papilla lie usually five, exceptionally six, irregularly pear- 

 shaped single-celled glands, in which a nucleus is usually visible, the extremely 

 fine ducts of which, slightly broadened at the mouth, run out longitudinally between 

 the processes of the elongated epithelial cells. According to Reichensperger, these 

 gland cells measure in Antedon mediterranea 0.006 mm. to 0.008 mm. by 0.002 

 mm. to 0.003 mm. ; the leVigth of the ducts varies according to the state of contrac- 

 tion of the papillae. 



Longitudinally through the center of each papilla there runs an axial muscle 

 fiber bearing a prominent nucleus, which is produced inwardly into the epithelial 

 muscle layer of the tentacle. 



Though the relative size varies, the structure of the tentacle papillae is the 

 same in Isocrinus and in the Comasteridae as in Antedon, and in these types also 

 five is the usual number of gland cells present about their bases. 



The structure of the tentacle papillae has been interpreted in a number of 

 different ways, giving rise to a considerable amount of controversy. 



Wyville Thomson described them as hollow, their lumen in communication 

 with that of the tentacle. He noticed that they are arranged in three or four 

 irregular longitudinal rows on each tentacle, and that they are extensible, when 

 extended appearing uniform and structureless, when contracted showing three or 

 four annular grooves. 



Perrier denied the existence of a median cavity in the papillae in connection 

 with that of the tentacles, and described in its place a glistening muscle fiber which 

 is produced into the second layer of the tentacle wall. On each of the three ter- 

 minal lappets he noticed an extremely fine, stiff hair, which he said disappeared 

 after death. 



Ludwig described the wall of the papillae as uniform and structureless. Deny- 

 ing the presence of a central muscle fiber, he considered that the median axis was 

 occupied by a longitudinal canal, which broadens out in the epithelial laver of 

 the tentacle into a spherical cavity which is entirely cut off from the lumen of 

 the tentacle. He comments upon the extraordinary extensibility of the papillae, 

 which in Heliometra glacialis measure when extended 0.063 mm. in length by 



