DEVELOPMENT. 



GO 



-our only rcpvcsentativc of the Eleulhero- 



Fig. 32. 



The development of the hyclranth-ljiul in Ui/di 

 blastea — seems to be in all essential 

 points the same as in the Gymnoblasiea, 

 the most important dififerences being 

 those which depend on the absence of 

 a perisarc in Hydra. The ultimate 

 destination of the bud, however, is very 

 different in the two cases ; for while in 

 the Gymnoblasiea it remains fixed as a 

 permanent part of the hydrosoma, it is 

 in Hydra destined to become detached 

 and enjoy henceforth an independent 

 existence.^ 



Sponianeous Decapitation and 

 Tie-formation of successive Hydranths. 

 — Our account of the development of 

 the hydranth-bud in the Gymnoblastea 

 would be incomplete without some re- 

 ference to a very remarkable pheno- 

 menon presented by certain species 

 of Tubularia, namely, the periodical 

 shedding and renewal of the hydranths. 

 This phenomenon was several years 

 ago observed by Dalyell," and described 

 with all his usual accuracy by this 

 excellent observer. I cannot find, 

 however, that any author has followed 

 the process with that exactness which 

 is necessary to enable us to form a 

 correct idea of its nature. My own 

 observations have been principally 

 made on Tubularia indivisa, where I 

 have bestowed upon the process in 

 question a very careful examina- 

 tion. 



When the hydranth of this species, with its clusters of gonophores, has acquired full 

 maturity, the time is come when it is to be cast off (woodcut, fig. 32 A), and its place taken by 

 a successor. A breach of continuity now occurs in the endoderm of the stem at a short distance 

 behind the hydranth ; while the ectoderm (c) having already become detached from the endoderm [h) 



Shedding and Renewal of the Hydranth in Tubularia indivisa. 



A, Part of a stem in which a breach of continuity has just taken place 

 below the hydranth ; a, the perisarc, closely investing the proximal seg- 

 ment of the stem j 6, the endodermal tube of the distal segment, which 

 has become separated and retracted from the proximal segment and from 

 its own ectoderm ; c, the detached ectoderm of the distal segment ; d, the 

 perisarc, where it forms a loose membranous sheath, just below the old 

 hydranth. 



B, Early stage of the new hydranth, with the commencement of the 

 proximal zone of tentacles; a, the perisarc, where it invests the stem just 

 below the new hydranth ; c, remains of the ectoderm, which had belonged 

 to the detached part of the stem ; d, remains of the perisarc, where it had 

 formed a loose, thin membranous sheath, below the old hydranth ; e, com- 

 mencement of new proximal tentacular zone ;_/, distal constriction, from 

 which the distal circlet of tentacles is to become developed ; g, the trun- 

 cated extremity of the proximal segment, now closed over, and containing 

 a cavity formed by the coalesced canals of the endoderm . 



C, More advanced stage of the new hydranth, showing the formation of 

 the second or distal circlet of tentacles ; a, perisarc, closely investing 

 the stem just below the new hydranth ; rf, remains of the distal part of the 

 perisarc, where it had formed a loose membranous sheath ; e, proximal zone 

 of tentacles, now much elongated j/*, distal zone of tentacles; g, terminal 

 part of the proximal segment, forming the hypostome of the new 

 hydranth. 



' The curious free reproductive bodies which occur in Corymorpha nutans, and which probably 

 originate as gemmae, though their exact significance has not yet been determined, will be described 

 below. 



° ' Rare and Remarkable Animals,' 1847, vol. i, p. ■4. 



