MONOGRAPH OF THE EXISTING CRINOIDS. 321 



of the dorsal portion of the chambered organ cirrus vessels are found which run 

 through the centrodorsal and end blindly within the epithelium; these are the 

 last traces of cirri which have become obsolete and have been discarded. The 

 median partition in each cirrus vessel is of the same structure as the septa of the 

 chambered organ, with which it is continuous. It consists of a thinner connective 

 tissue sheet covered on both sides with epithelium, and does not contain muscle 

 fibers. 



The cavities of the chambered organ are lined with an epithelium which is 

 continuous with and similar to that covering the radiating connective tissue 

 septa, and which is also produced into the cirrus vessels. This epithelium is 

 composed of uniform closely crowded cells 0.005 mm. to 0.004 mm. high with 

 deeply staining nuclei and without cilia, which form a continuous layer. 



Perrier noticed in the cavities of the chambered organ a great number of 

 free elements, very regularly spherical and staining only very faintly. He never 

 found these bodies outside of the chambered organ. 



Eeichensperger in his studies on Isocrinus noticed these bodies and later made 

 a detailed investigation of them in several species of comatulids. 



In all parts of the chambered organ, and only within that structure, large 

 and small spherical bodies occur which show a decided partiality for certain stains. 

 The small particles are composed of a fine friable substance ; the larger are covered 

 with the same material, some with a very thin, others with a thicker stratum. 



These spherical bodies are hollow spheres, of which the largest measure from 

 6(x to 10 [i. in diameter. They were found in all the species examined. 



Eeichensperger was not able to determine the place of origin of these spheres 

 but suggests that they may possibly be formed by the moderately high epithelium 

 of the chambered organ, for on this epithelium lie most of the smaller bodies, as 

 well as those which are more thickly enveloped in organic substance. He did not, 

 however, observe actual origin from the epithelium, or any alteration within its 

 cells which would suggest such an origin. The minuteness of the elements involved 

 rendered determination of this point difficult. 



These bodies were found in the following material : 



Five Isocrinus deconis, whole chambers with numerous little spheres. 



Two Comanthus bennetti, whole chambers with numerous little spheres. 



One Comasterid, species undetermined, the same. 



Six Antedon mediterranea, the same. 



Of three specimens of Antedon mediterranea which were regenerating whole 

 arms one possessed a very few little spheres, the two others none at all, but instead 

 there appeared in the chambered organ a network of a strongly stained substance 

 resembling that enveloping the spheres. 



Keichensperger suggests that these spheres may serve to contain a reserve 

 supply of limestone in a more or less concentrated solution for use in the reparation 

 of the skeleton. 



The chambered organ, together with the canals which lead into the cirri, in 

 the very young animal is in communication with the body cavity, of which it is 

 an integral part, but from which it later becomes entirely cut off. 



