350 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Carpenter not infrequently found in the disk of the same species that the 

 lumen of the canal may be crossed in various directions by delicate threads, with 

 nuclei in them, which resemble the nucleated muscle threads in the water vessels, 

 but are much finer and less refractile. 



Hamann believes that the canal in which the genital tube lies may possibly 

 be a schizoccele canal. It is lined with a flattened endothelium, the cells of which 

 in size and appearance resemble connective tissue cells. This endothelium is 

 only feebly developed and may be partially deficient. In any case, the cells are 

 to be differentiated markedly from the cells which line the canals of the arms. 



Ludwig and P. H. Carpenter considered the schizoccele canals of the arms 

 the radial canals of the blood-vascular system. 



Perrier denied their existence altogether. 



Hamann says that these cavities are not connected with the blood lacunar 

 system, and are usually filled with a clear fluid like that of the body cavity. 



Eeichensperger at first considered the schizoccele canals as blood vessels, fol- 

 lowing Ludwig and P. H. Carpenter. Later he admitted that the contents of 

 these cavities indicate that they undoubtedly perform the functions of blood 

 lacunae, since no other cavities of the arms contain so many lymph cells, which 

 are inclosed in typical blood channels. 



He now considers that, although on account of their origin and form, and 

 on account of the absence of a uniform endothelium, they can not be considered 

 as normal blood vessels, the schizoccele canals are in reality blood vessels derived 

 through what are primarily schizocffile structures. 



CILIATED EPITHELIUM OF THE AMBDLACBAL GttOOVES. 



The ciliated epithelium of the ambulacra! grooves is composed of two kinds of 

 cells, epithelial sense cells, and supporting cells, the former in the majority. Both 

 types bear cilia which are attached to the slender cell body by foot pieces. 



The epithelial sense cells are fine and almost hairlike. The spindle-shaped 

 cell body, inclosing an elongate-oval nucleus which shows a distinct network, is 

 produced outward into a fine process of variable length on which a cilium stands ; 

 inwardly it is produced into a fine extension, sometimes swollen here and there, 

 which may be followed into the nerve layer. 



The supporting cells, possessing also elongate-oval nuclei which appear to stain 

 less deeply than those of the sense cells, are produced interiorly into stout hj'aline, 

 strongly refractile processes. 



The epithelium lining the depression about the mouth opening is a continuation 

 of the epithelium of the ambulacral grooves and is composed of the same elements. 

 Near the ends of the arms and pinnules this epithelium disappears. 



CILIATED SACKS. 



Under the name of ciliated sacks Ludwig described in the dorsal wall of the 

 extension of the body cavity into the pinnules sacklike invaginations grouped 

 in each pinnule segment which are lined with ciliated epithelium. 



Hamann studied these structures in detail in Comatula solans, Comanthus 

 parvicirra, Antedon mediterranea, Heliometra glacialis. and Isocrinus decorus. 



