REPRODUCTION. 153 



elongate itself. In doing so it ruptures the delicate pellicle of chitine which closes the extremity 

 of the ramuhis, and extends itself quite naked into the surrounding water. 



It is now that the process of fission commences. A constriction takes place in the coenosarc 

 at some distance below its distal extremity, and in the part still covered by the chitinous peri- 

 sarc [b). The constriction rapidly deepens, and ultimately cuts off a piece (c), which slips 

 entirely out of the perisarcal tube and becomes a free zooid {d), while the surface of disseveration 

 soon heals over, and the axial cavity of the free frustule becomes here as completely closed as at 

 the opposite end. 



The detached segment is now about the yj]^ of an inch in length, and strikingly resembles 

 a planula in all points except in the total absence of vibratile cilia. It attaches itself by a mucous 

 excretion from its surface to the walls of the vessel, and exhibits slight and very sluggish changes 

 of form. It now slightly advances along the surface of support, withdrawing itself from the first- 

 formed portion of the excretion, which remains behind as a tube of great tenuity, adhering to the 

 sides of the vessel (/). 



In tracing the further history of the frustule it was foiuid that this never directly developes 

 a mouth or becomes transformed into a hydranth. After a time a bud springs from its side (y), 

 and it is from this bud alone that the first hydranth of the new colony is developed. 



The bud which thus becomes developed into the primordial hydranth remains attached to 

 the fission-frustule, which forms for it a sort of hydrorhiza, but which would seem ultimately to 

 perish and give place to true hydrorhizal filaments. In the mean time the primary bud emits 

 others (e), and a complex branching colony is the result. 



The fission-frustule thus admits of a comparison with the free medusiform element of other 

 hydroids, with which it agrees in never becoming directly developed into a hydriform trophosome, 

 but from which it differs in the very important fact of taking no part in the true generation of 

 the hydroid, and in giving origin to a new colony only by a non-sexual multiplication. 



The fissiparous multiplication of Sckizodadium Avould seem to throw light on the nature 

 of certain bodies which made their appearance in a jar containing living specimens of Cori/morpJia 

 nutans (see Plate XIX, figs. 12 — 14). These bodies presented a close resemblance to the fission- 

 frustule of ScMzocladlum , and were seen to become developed into hydranths, which it is almost 

 certain ultimately repeat the form of the adult Corymorpha. Their origin was, at the time I 

 noticed them, very enigmatical, but I now regard it as highly probable that they are produced 

 by a process of spontaneous fission from the filaments which are emitted towards the base of the 

 stem in the Corymorpha. They would seem, however, to differ from the fission-frustules of 

 Scltizocladium in becoming directly developed into a trophosome. 



The decapitation and successive renewal of the hydranths, referred to above (p. 69) as 

 occurring in various species of Tuhalaria, may be compared with the phenomenon of fissiparous 

 multiplication just described. In the decapitation of Tuhularla, however, the separated hydranth 

 is not destined to undergo any further development ; it has matured its sexual buds, and has 

 accomplished all the objects of its existence before being cast off, and it then perishes, to be 

 replaced by another. 



The decapitation of Tiihiduria admits of a still closer comparison with the formation and 

 detachment of discs {cphyrai) from the hydriform stage [scypliostoma) of Aurelia and other 

 BlscopJiora. Here, however, the discs into which the scypliostoma breaks up by a process of 

 transverse division which has its equivalent in the budding, by which the sexual zooids are formed 



20 



