344 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



P. H. Carpenter noted that the presence of tentacular branches from the water 

 vessels is invariably correlated with the presence of an ambulacra! groove on the 

 ventral surface of the arms and pinnules. If this remains undeveloped not only 

 are the ambulacral epithelium, nerves, and blood vessels absent, but, since there are 

 no tentacles, the water vessels are simple tubes without lateral extensions. 



This condition may occur in a majority of the arms and the corresponding 

 portions of the disk in many of the species of the Comasteridse, in a variable num- 

 ber of the lower pinnules in certain of the Comasterida) and (^haritometridse, and is 

 found in the proximal pinnules of most comatulid types. 



On the other hand, P. H. Carpenter noticed that in Metacrinus the ambulacral 

 grooves, and with them the water vessels of the large basal pinnules, may arise 

 directly from the primary ambulacra of the disk, or even from the peristome itself. 



In a cross section of an arm one sees in the floor of the ambulacral gi'oove the 

 high epithelium of thread-like cells, between the basal processes of which run the 

 epithelial nerve fibrillse; beneath this epithelium with its nerve fibrillar layer lies 

 a thin connective tissue layer in which a schizoccele cavity can be made out: and 

 just below this lies the water vessel, giving ofl^ at intervals to right and left smaller 

 vessels to the tentacles. The water vessel often lies so closely to the schizoccele 

 cavity in the connective tissue layer just above it that the partition separating 

 them appears merely as a thin membrane of firm consistency. The water vessel 

 and the overlying structures maintain the same relationships on the disk. 



The water vessels are lined with a low epithelium composed of cubical cells. 

 Each cell of the external layer under the epithelial nerve is prolonged into a muscle 

 fiber which runs parallel to the axis of the vessel, and is therefore an epithelial 

 muscle cell. 



These epithelial muscle cells have been found by Hamann in Gomatula rota- 

 laria, Comanthus parviciiva, Tropio7netra picta, Antedon ?nediterra?iea, and Iso- 

 crinus decoi-us. In Antedon mediterranea the height of these cells is 0.003 mm., in 

 the tentacles 0.004 mm. They bear no cilia. 



An exceedingly fine membrane ensheathes the water vessels. 



The lumen of all the water vessels is traversed by muscle fibers, and there are 

 also longitudinal muscle fibers attached to the epithelial cells. 



The canals which open into the chambers of the body cavity from the circumoral 

 canal are in Antedon mediterranea 0.002 mm. in diameter. They are circular in 

 cross section and are composed of three layers, a layer of ciliated epithelium lining 

 the inner cavity, a thinner connective tissue layer, and an external epithelium, 

 which at the openings passes over into the ciliated epithelium. The inner layer is 

 composed of cells the nuclei of which are elongated oval in form ; they possess so 

 little cytoplasm that in a section the layer appears as a closely crowded layer of 

 nuclei. The length of these canals in Antedon mediferranea is 0.2 mm. 



At their free ends these canals are almost always bent ; their mouths are fimnel- 

 shaped, and the same in Tropiometra picta as in Antedon viediterranea. The cilia 

 are always very finely preserved. The thin connective tissue layer in their walls 

 shows no trace of calcification . 



