1895. PROCEEDINGS OF THE NATIONAL MUSEUM. 739 



observations the hydranths were simply placed in a little sea water 

 under cover glass and gently flattened. A point was selected where a 

 nettling organ, not far from the base of a tentacle, was slowly chang- 

 ing its form. It was observed for nearly one-half hour, the ectoderm 

 cell boundaries being used as the nearest fixed points obtainable. The 

 large mass of protoplasm containing the nucleus of the uematoblast 

 was turned toward the tentacle. At the end of the stated time of 

 observation, the organ had passed through a distance e([H'd\ to its own 

 diameter. In another case a nettling organ traveled toward a tentacle 

 a distance equal to three times its diameter; meanwhile it twice turned 

 np endwise. Another case particularly drew attention ; the cell-body 

 was changing its form quite rapidly, progressing at the same time 

 between the ectoderm cells, keeping close to the mesoglcea. Many 

 cases were observed where the nettling organs were lying parallel to 

 the surface of the tentacles. 



Other cases were observed where the nettling organs were turned in 

 almost any direction, or again w^xere they seemed to be reversed as if 

 going toward the base of the tentacle, and many others in which I 

 could detect no change of form or motion whatever. These exceptions, 

 however, as well as the short distance traveled by the nettling organs 

 in a given time, may find an explanation in the abnormal conditions to 

 which the hydranths were subjected during the observations. 



After the foregoing observations I feel warranted in reaffirming my 

 previous conclusion, that the active amoeboid migration of the nettling 

 organs Ls the manner in which they are transported from the point of 

 their development to their destination. Furthermore, I believe this will 

 be found to apply also to all Onidaria where similar conditions obtain 

 as, for instance, to all except some of the Siphonophora. 



In the limits of this paper it is not expedient to give a review of the 

 literature on the origin and development of the nematocystand thread. 

 Suffice it to say that most of the authors heretofore agreed that the 

 uematocyst and contents take their origin from a vacuole arising in the 

 protoplasm of the nettle-cell. On the origin of the thread all the 

 older authors are agreed that it arose in the uematocyst; some, from a 

 mass of protoplasm that grew into the vacuole, and others believed it 

 origiimted in the secreted contents of the uematocyst. Still another 

 view was that both uematocyst and thread were derived from the mass 

 of protoplasm that had grown into the vacuole. But later it was 

 finally shown that the thread takes it origin outside the uematocyst, 

 and consequently it must subsequently be invaginated into the cap- 

 sule. 



With the exception of one brief reference,^ this fact in regard to the 

 position of the growing thread was not applied to the Actiniic. It 

 was therefore desirable to learn to what extent my observations ou 

 Hydroids aj)plied to these. 



' Schneider, Einige liist. Befnnde au Cct'lenteraten ; Jen. Zschr. f. Nat. 27, N. F. 20, 

 1892. 



