14 



Sacculi. 



The sacculi (PI. II., figs. 24 and 28; PL III., figs. 32, 

 33, 34, and 40; PI. lY., figs. 41 and 47; PI. V., fig. 52; 

 PI. YIL, figs. Gl, 62, 66, 67, 69, and 71, sac.) are minute 

 globular or ovate sacs, which occur in great numbers 

 immediately below the external epithelium at the edges 

 of the ambulacral grooves of the disc, arms, and pinnules, 

 and occasionally in small numbers in the wall of the 

 intestine and mesenteries. Along the grooves of the disc 

 they may form a double or even a triple row, and are 

 especially numerous on the outer sides of the proximal 

 ends of the brachial grooves, on the periphery of the 

 disc (fig. 24). Along the arms and pinnules they form 

 a single row, and alternate regularly with the triad groups 

 of tentacles. Each sacculus consists of a delicate limiting 

 membrane of connective tissue, lodgecl in the mesoderm 

 and having no permanent aperture (PI. III., fig. 40). The 

 included space is almost invariably densely crowded with 

 groups of refractive spherules, consisting of an albuminous 

 substance, which are colourless during life, but turn 

 yellow or red after death, owing to absorption of the very 

 soluble pigment of the integument. They have a strong 

 affinity for stains. 



Tliese spherules are the product of nucleated cells of 

 pyriform shape (fig. 40, rr), which clothe the inner face 

 of the lower wall of the sacculus, and are apparently 

 of mesodermal origin. The spherules make their appear- 

 ance in processes of the cells which grow upwards and 

 finally become attached in the form of filaments to the 

 upper wall of the sacculus. The spherule-containing 

 portion eventually separates from the base of the cell and 

 lies free in the sacculus. Sacculi have been successively 

 regarded as calcigenous glands, mucous glands, excretory 



