348 BUL1.ETIN 82, UNITED STATES NATIONAL MUSEUM. 



vessels in the upper surface of the visceral mass from which are derived both the 

 intervisceral vessels and the genital vessels of the two posterior ambulacra. 



Hamann considers the differentiation of the labial plexus and the spongy organ 

 from the remaining portions of the blood vascular system impracticable and 

 unnecessary. 



SCHIZOClELB CAVITIES. 



In the crinoids beneath the ambulacral groove of the arms and pinnules and 

 just below the epithelial nerves there runs a slitlike canal which may, though 

 very rarely, be made out as traversing the disk and forming a ring around the 

 mouth. 



In Antedon mediterranea Hamann found that this schizoccele canal runs out 

 into the arms as a constant structure. It may be closed here and there through 

 the contraction of the musculature of the water vessels in the ambulacral groove, 

 for just as soon as the water vessels become narrowed through the action of the 

 transverse muscle fibers the schizoccele canal vanishes. When the water vessels 

 are thus contracted the epithelium of the ambulacral grooves is not so much arched 

 as normally, forming instead, as seen in cross section, a straight line. The schizo- 

 ccele canal may also be closed by the movements of the arms. 



In Antedon mediterranea the longitudinal schizoccele canal of the arms appears 

 in cross section as an oval cavity of small size, 0.04 mm. to 0.09 mm. in diameter 

 according to Reichensperger, and on well-preserved sections of the pinnules as 

 well as of the arms it can regularly be made out. 



In Heliometra glricialis it has a diameter of 0.1 mm. It is slitlike to circular 

 in cross section, according to the state of contraction of the musculature of the 

 water vessels. 



In large species like the latter the schizoccele canal, according to Hamann, 

 shows an endothelium of flattened cells 0.01 mm. in height, the nuclei of which 

 project into the lumen, making it evident that the cavity can not be an accidental 

 gap in the connective tissue layer beneath the epithelial nerve, but must be instead 

 a true and permanent cavity. 



Reichensperger, however, was unable to convince himself of the existence of 

 the lining of flattened cells described by Hamann. 



P. H. Carpenter notes that the schizoccele canals are large in Heliometra 

 glaciaUs and in ComateMa nigra. In the latter they are sometimes triangular in 

 section with the apex pointing downward, so as to be received into a strongly 

 marked concavity in the upper edge of the water vessel ; but in Heliometra glacialis 

 and in most other types their section is more or less lenticular, though sometimes 

 triangular with the apex projecting upward toward the epithelial layer above, and 

 so rendering the ambulacral nerve thinner in the middle line than in its more lateral 

 portions. He further remarks that the cellular lining of these radial canals is 

 much more delicate than that of the intervisceral blood vessels and is not easy 

 to make out. 



Ludwig found that the schizoccele canals in the arms of Heliometra glacialis 

 are sometimes divided into two parts by a vertical septum which has a distinctly 

 cellular covering. 



