DEDIFFERENTIATION IN HYDROIDS AND ASCIDIANS* 

 By H. V. Wilson 



Eudendrium colonies {E. carneum Clarke) kept in aquaria, at 

 Beaufort, N. C, during August (1922), dropped their hydranths in 

 the course of twenty-four hours, new ones in many cases being later 

 regenerated from the ends of the hydrocaulus system. Before the 

 hydranth is constricted ofl the tentacles shorten and fade away into 

 the body, the mouth closes, and the demarcation betw^een hypostome 

 and the rest of the hydranth disappears. The hydranth is thus con- 

 verted into a smooth, ellipsoidal, thin walled, and often swollen sac, 

 which drops off. These ' ' planuloid ' ' bodies in the actual experiments 

 died without evidencing regenerative power. In July (1910) hy- 

 dranths of the same species, cut off at the base from the hydrocaulus 

 and kept in aquaria, Avere observed to undergo in about the same time 

 a similar series of dedift'erentiative changes ; the resulting planuloids 

 were globular. 



This series of changes is closely parallel to that exhibited by starv- 

 ing hydras (Schultz, Archiv f. Entw.-Mech. 1906), except that testes 

 commonly develop in the latter. The changes as recorded for other 

 hydroids are somewhat different. In Campanularia (Thacher, Biol. 

 Bulletin, 1903) the hydranth, w^hile going through much the same 

 anatomical phases, retains its connection with the stem, into the gastric 

 cavity of which its tissues, in the shape of degenerating and dissociated 

 cells, pass. Thus ultimately the whole mass of the hydranth is used 

 up as food. Thacher finds that essentially the same process of de- 

 generation and absorption as food material is undergone in other 

 Woods Hole species (Eudendrium and Pennaria). In Tuhnlaria 

 (Morse, Biol. Bulletin, 1908-09), at Woods Hole, the hydranths fall off 

 in confinement while yet practically normal in appearance. 



What in a gross way, at least, presents itself as a case of "reduc- 

 tion" may also be observed in male gonosomes of Eudendrium car- 

 neum. A male gonosome consists of the expanded end of a branch, 

 representing an aborted hydranth, bearing a whorl of radially pro- 



* I am indebted to the U. S. Commissioner of Fisheries for a working place in the 

 Laboratory of the U. S. Bureau of Fisheries at Beaufort, N. C, where the observational 

 work recorded in this paper was carried OJi. 



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