21 



The Axial Organ. 



This organ, variously known as " dorsal organ," 

 " glandular organ," and " genital stolon," has already 

 been seen in connection with the chambered organ (PI. V., 

 fig. 52; PL VI., fig. 59, aa;. or.). That portion which 

 forms the axis of the latter consists of a comparatively 

 slender cord, traversed by a few tubules lined with 

 epithelium (fig. 59), which finally merge into one in 

 the direction of the centro-dorsal (PI. VI., figs. 54, 55, and 

 57, ax. or.). In the opposite direction the organ passes 

 through the central foramen of the rosette plate, ascends 

 alongside the axial sinus of the coeloni towards the mouth, 

 and, in the adult animal, ends in a rounded extremity 

 some little distance below the tegmen calycis, in the right 

 anterior radius. This portion of the organ consists of a 

 complex mass of tubules lined with cylindrical epithelium 

 (PL IV., fig. 46), and enclosed in a stroma of connective 

 tissue. As already stated above, a part of the lacunar 

 system is in close relation with the axial organ ; but the 

 cord-like extensions of its free (oral) end w^hich have been 

 described as passing into the arms of the pentacrinoid 

 larva, there to form the genital rachids, do not persist in 

 the adult. A departure from the structural condition just 

 described appears to be associated with the breeding 

 season. The lumina of the tubules open into one another 

 much more freely than at other times (PL IV., fig. 44), 

 and the epithelial cells which line them break away from 

 the basement membrane and become amceboid. Various 

 stages of this process may be seen in serial sections of one 

 and the same organ. 



