324 



Keichensperger reinvestigated the axial organ in great detail but did not 

 succeed in elucidating its relation to the other organs further than to convince 

 himself that it is in intimate relation with the blood lacunar system, as had been 

 assumed by Ludwig and more recently reaffirmed by Chadwick. 



He found many times places where the blood vessels apparently enter directly 

 into the organ itself, the thin epithelium of the blood vessels passing directly 

 over into the thicker epithelium of the axial organ. In addition, he sometimes 

 found in the blood vascular system the products of the cells of the axial organ, 

 in the shape of rounded granules. 



That these rounded granules are the product of the cells of the axial organ 

 Keichensperger believes is shown by the fact that in the places where the granules 

 lie the cells have a markedly glandular character. Among other features, the 

 cytoplasm shows small vacuoles. 



Chadwick noted that at the breeding season the cells of the axial organ in 

 Antedon lifida loosen from their basal membrane and become amoeboid. 



Keichensperger was not able to confirm this in Antedon mediterra'nea, although 

 he was at Naples during the height of the breeding season. 



CENTRAL 01H5AN. 



The central organ of the dorsal nervous system, sometimes known as the 

 central capsule, is composed of a mass of nerve fibrillse, in general concentric 

 in arrangement, which entirely envelops the chambered organ except for a small 

 area at the ventral pole where its axis passes ventrally into the axial organ. 



From the dorsal portion of the central organ, involving all of the nerve fibrillar 

 layers, arise radially the envelopes of parallel nerve fibrillse which ensheath 

 throughout their whole course the tubular extensions of the cavities of the cham- 

 bered organ into the cirri. 



Above these, with their bases in contact all around the central organ, are 

 given off the five large and stout primary nerve branches which, narrowing and 

 passing diagonally upward, almost immediately divide, the two resultant halves 

 of each diverging widel} 7 and merging with the similarly diverging halves of the 

 two adjacent to form, within the radials, the secondary nerve trunks which become 

 the dorsal nerve cords, or axial cords, of the arms. 



The inner fibrillse of the central organ, which lie directly upon the chambered 

 organ, run concentrically, this arrangement being interrupted only at the bases of 

 the cirrus vessels. Those of the cortical layer are for the most part concentric, 

 but their arrangement is very irregular at the bases of the cirrus nerves. 



The fibrillae of the five nerve masses which form the five primary nerve trunks 

 are parallel. 



Large ganglion cells are especially numerous on the dorsal side of the central 

 organ at the base of the cirrus nerves, and where the five primary nerves arise, 

 and these still further interrupt the normal concentric course of the fibrillae. 



CIRRUS JJEKVES. 



Each of the tubular vessels which run from the chambered organ into the 

 cirri is incased all along its course by an investment of parallel nerve fibrillii- 



