THE PLUMULARIDJE. 



arises. ' And again the hydrocladia become atrophied, lose their Characters as hydrocladia, and 

 become transformed into tubes which support the original stem from which they spriug. 



Hydrant/in. Those of the Plumularida- are of uniform structure throughout the group, so far 

 as I have been able to ascertain. It should be noted, however, that they are usually absent or 

 partially disintegrated in many alcoholic specimens, and of course in all dried material. The 

 examination of living and expanded hydranths of numerous species has not resulted in the discovery 

 of any considerable departure from the, type ordinarily described in systematic works (fig. 7). 

 The tenta.cles are always solid, filiform, arranged in a single whorl, usually of from eight to sixteen, 



HYDRANTH OF PLUMULARIA TRII'ARTITA. 



Fig. 7. (After von Lentlenfelil) c, body cavity ; ect, ectoderm; end, endoclerm : 7i, liyilrotla'i :i : >. mmith : n, sarcostyle. 



but sometimes as many as twenty four. The proboscis is conical or dome-shaped. The hydranth 

 usually has a more or less evident constriction which divides the body cavity into a distal and a 

 proximal portion communicating broadly, otherwise the hydranths of this group can not be easily 

 distinguished from many found among the Sertularida- and (.lampanularidiv, with which they agree 

 in arrangement of tentacles, general form, microscopic structure, and relation of histological layers. 

 Where the hydrotheca has a pronounced intrathecal ridge, there is a bend in the body of the 

 retracted hydranth to accommodate the latter to the ridge ; but this does not seein to att'ect the 

 contour of the expanded polyp. 



The tentacles of the liydranth are armed with rather small nematocysts; but these do not 

 seem to be as frequently brought into use as in other groups, perhaps on account of the special 



1 See discussion of the homology of the accessory tubes in the fascicled steins, p. 6. 



