THE CRINOIDEA 



137 



the inner side of the coil is thrown into folds or villi (Fig. LIV.). 

 In Actinometra, however, with its long coil, such plication is slight. 

 Sacculi (Fig. LV.) are structures confined to this class. They 

 are globular sacs surrounded by mesoderm, but lying close beneath 

 the epithelium, usually of the external surface. The lower wall 

 of each sacculus is clothed with rather large nucleated cells, 

 apparently derived from mesoderm, and from these grow up 

 processes filled with refringent granules of albuminoid substance. 

 Each process elongates and becomes attached to the upper (i.e. 

 outer) wall of the sacculus by a filament. The granular portion 

 of the cell may then separate from the nucleated base, and finally 

 may burst, setting free the granules. These granules, colourless 

 in life, are stained on death by the yellow pigment of the peri- 

 some, and show strong affinity for most staining reagents. The 



*~IBr. 



FIG. LIII. 



Section across the thecal 

 cavity of Bathycrinus Al- 

 drichianus, at the level of 

 second primibrachs, show- 

 ing interradial processes of 

 stomach (st), the rectum (r), 

 and the axial organ (a*). 

 (Diagrammatised from P. 

 H. Carpenter.) x 1 diam. 



FIG. LIV. 



Longitudinal section of 

 wall of anal tube of Antedon 

 bifida, showing villi (v) of 

 rectal wall and their gland 

 cells (g). In the connective 

 tissue layer arc plates of 

 stereoni (st), and outside is 

 epithelium (e). (Diagram- 

 matised from Hamann.) 

 Greatly magnified. 



FIG. LV. 



Section of a nearly ripe sac- 

 culus of Antedon bifida. int, 

 superficial layer of integument ; 

 /, librillar attachments of con- 

 tained cells to integument ; cut, 

 cutis surrounding sacculus, and 

 containing nuclei (ns) ; g, granu- 

 lar masses of the contained cells ; 

 the large nuclei of these cells are 

 seen below in the lining epi- 

 thelium (ep) of the sacculus. 

 (After Cuenot.) x 200 diam. 



sacculi occur chiefly at the edges of the food-grooves ; and the 

 side-plates, when highly developed, are notched for their reception 

 (as seen in Fig. IX. 2). They have also been observed in the 

 walls of the gut, in the mesenteries, and above the chambered 

 organ. They are developed so soon as the larva begins to feed. 

 Sacculi have been regarded as calcigenous glands (Wyville Thomson), 

 mucous glands (Bury), excretory organs (Ludwig), symbiotic algae, 

 " Zooxanthellae " (Vogt & Jung), and accumulations of reserve 

 material (Walther, and especially Cuenot, 1891). Since the com- 

 prehensive account by the last-named, little has been written on 

 the subject, and his view has found general favour. Sacculi occur 

 in Antedon, Promachocrinus, Eiidiocrinus, Atelecrinus, Rhizocrinus, 

 Bathycrinus, and Pentacrinus ; they are certainly absent from Actino- 

 metra, and probably from all other recent genera. This fact renders 

 their occurrence of taxonomic value. 



