322 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



AXIAL ORGAN. 



The axial cord which traverses the chambered organ emerges from it ventrally 

 and continues as an irregular cord-like structure into the calicular body cavity, 

 running upward between the coils of the digestive tube and ending blindly in the 

 vicinity of the mouth. It does not lie in the center of the calyx, but on one side. 



The axial organ is covered on all sides with coelomic epithelium composed of 

 squarish or more flattened cells. A cross section through it shows a series of 

 chambers lying in a common connective tissue, which is distinct from the crelomic 

 epithelium. The nearer the sections studied lie to the ventral surface of the 

 animal the more numerous are the cavities, some of which are cut across, others 

 longitudinally. 



Toward the middle of the organ the diameter and the number of the chambers 

 increases, from that point onward gradually diminishing again. Longitudinal 

 sections show that these chambers give rise to short, blind, lateral branches, the 

 greater part of which run longitudinally. Single chambers may branch out 

 together from the main mass, causing the organ, originally in a single line, to 

 break up into several lines. These branches are also covered with coelomic 

 epithelium. 



The individual chambers are lined with an epithelium about 0.04 mm. in 

 height composed of closely crowded cylindrical cells showing granular contents, 

 and with the circular vesicular nuclei lying at their bases. The lumen of the 

 chambers is sometimes wide and sometimes narrow, and is often filled with a 

 coagulable clear fluid. 



The connective substance in which all these chambers lie is made up of clear 

 ground substance which shows no calcification, and spindle or star shaped cells 

 the processes of which are irregularly intertwined. 



The circumference of this organ decreases toward the ventral side, and the 

 last chambers end blindly. The dorsal end dwindles away, and is continued as 

 the axial cord of the chambered organ. Sections through this end portion show 

 that the number of the chambers has become very small and decreases dorsally. 

 The chambers are very narrow and possess an epithelial lining which differs 

 from that in the chambers lying within the body cavity, being composed of lower, 

 almost flattened, or cubical cells. 



The last four or five narrow little canals end blindly. The cirrus vessels are 

 not processes from these canals, but arise from the chambered organ. 



According to Hamann the axial organ is a closed capsule ensheathed by fibrous 

 connective tissue. The inclosed vesicles, which have no connection with the 

 exterior, are shorter or longer tubes, sometimes flask shaped, sometimes club 

 shaped and partially branched. 



Hamann noticed that blood lacunae may be traced into the immediate vicinity 

 of the axial organ, and that they appear narrowly to connect with it on the 

 ventral end. The blood from these lacunae can not enter the lumina of the cavities, 

 but can at the most enter the connective tissue of the organ, where, however, 

 Hamann has not been able to observe it. 



