20 



arrangement, in close proximity to the dorsal nerve cord, 

 is probably not without signiiicance, though no nervous 

 connection between the two has been traced. 



The Chambered Organ. 



This organ (PL V., fig. 52 ; PL YL, figs. 54, 55, 57, 

 59, ch. or.) is a division of the coelom, and is lodged for the 

 most part in the cavity of the centro-dorsal plate. It 

 consists of five radially situated chambers of roughly 

 equal size, lined by coelomic endothelium, and completely 

 enveloped on all sides by the central capsule of the apical 

 or aboral nervous system (fig. 57). In the vertical 

 axis, from which the connective tissue septa which divide 

 the chambers spring, there is a column-like prolongation 

 of the axial organ (PL V., fig. 52 ; PL VI., fig. 59) which 

 penetrates the central capsule and gradually tapers off in 

 the substance of the centro-dorsal. Tubular extensions 

 of the radial chambers, known as cirrus vessels (figs. 52 

 and 59, ci. v.), enclosed in corresponding extensions 

 of the central capsule, called cirrus cords {ci. cd.), and 

 divided throughout by a horizontal septum (figs. 58 and 

 59) traverse the thickness of the centro-dorsal and the 

 axial canals which have been described above as running 

 through all but the terminal joints of the cirri. The 

 horizontal septa cross the chambers, some to be inserted 

 in the vertical axis (PL YL, fig. 59), while others do not 

 reach it. They thus appear in sagittal and tangential 

 sections to divide the aboral ends of the radial chambers 

 into a number of super-imposed spaces. On the other 

 hand, the septa are not continued to the extreme ends of 

 the cirrus canals, so that a circulation of the coelomic fluid 

 therein is possible. 



