THE VOYAGE OF H.M.S. CHALLENGER. 
xliv 
The transitory arms like the permanent tentacles are hollow. They terminate each 
in a well-defined capitulum and are about twenty in number. They form no regular 
verticil but extend with a scattered disposition over the greater part of the body, 
Upon the escape of the Actinula they continue to increase in length, and have the 
faculty of holding on by their clavate extremities to neighbouring surfaces. It is thus 
by their aid that the larva creeps about on the various objects in its vicinity. 
After the Actinula has enjoyed for some days its free locomotive existence, it begins 
to fix itself by its aboral sucker-like extremity ; the permanent tentacles become more 
numerous and extend further backwards on the body, while the long arms undergo 
a rapid degradation, become much shortened, and soon entirely disappear. All the 
essential features of the adult trophosome are thus acquired, and it only remains to 
complete the development of the Hydroid by the formation of the gonosome, which soon 
makes its appearance by the budding of the blastostyles and claspers from the hydranth 
at the proximal side of the tentacles. From the blastostyles the gonophores are 
subequally budded off, and the animal thus attains its complete maturity. 
Myriothela phrygia, the only known species of this remarkable genus, is monoecious, 
the same hydranth carrying both male and female gonophores, but at what part of the 
developmental process just described the male influence exerts itself, or what may be the 
immediate changes which result from this, are points on which the observed facts will 
not justify a definite conclusion. 
It will be seen that the formation of the Actinula in Myriothela is connected with 
certain phenomena which are very remarkable and exceptional. Among the most signi- 
ficant of these is the formation of a plasmodium by the coalescence of numerous primitive 
egg-cells. Exceptional, however, as this phenomenon is, it is not without a parallel 
even among the higher animals, and will at once recall the formation of the permanent 
ovum, as a syncytium, from the coalescence of the “ primitive ova” in the early stages of 
the ovary in certain Elasmobranch fishes . 1 
Development of the Egg in Hydra. — The phenomena presented during the develop- 
ment of Hydra are in many respects scarcely less divergent from the ordinary course of 
Hydroid development than those just described in Myriothela. 
It is to the researches of Kleinenberg that we are indebted for the first complete 
account of the development of the sexual structures in Hydra? He has shown that 
both ovaria and testes are derived from certain cells of the ectoderm which lie between 
1 See Balfour’s Works, Memorial Edit., vol. i. p. 587 ; vol. ii. p. 57. Korotneff, who seems to be the only other 
observer who has studied the structure and development of Myriothela ( Zool . Anzeig., Bd. i. p. 363, 1878 ; Bd. ii. 
P- 187, 1879) takes a different view of the whole process. He supposes that among the multitude of egg-cells which 
originally fill the cavity of the gonophore, only one becomes a true egg, and that from this alone the Actinula is developed. 
I cannot, however, accept this view. It is impossible to reconcile it with the facts just described, and a laborious investi- 
gation of the animal has, I believe, placed the truth of these facts beyond question. 
* Nicolaus Kleinenberg, Hydra, eine anatomisch-entwicklungsgeschichtliche Untersuchung, Leipzig, 1872. 
