570 
PROFESSOR ALLMAN ON THE STRUCTURE 
except Hydra , entirely passed within the gonophore, and the planula in such cases is 
never ciliated or locomotive. 
In Tubularia the planula is a non-ciliated compressed sac, developed directly out of 
the plasma mass which occupies the cavity of the gonophore ; while still retained within 
the gonophore it develops tentacles by outgrowths from its sides, elongates itself, 
becomes perforated by a mouth, and then escapes as a free locomotive actinula destined 
to undergo further changes of shape before attaining the final form of the hydroid 
trophosome. 
Just in the same way Myriothela passes through the non-ciliated planula stage before 
it attains the form of the free actinula. In one important point, however, the actinula 
of Myriothela differs from that of Tubularia, namely, in the possession of embryonic 
transitory organs which take the form of long contractile arms, by which locomotion is 
aided, and which entirely disappear during the subsequent course of the development. 
In Hydra, too, which never presents a permanently fixed trophosome, we find a true 
planula stage, the planula being here, as in the actinula-forming hydroids, destitute of 
cilia. It acquires a mouth by perforation, and develops itself by continuous growth 
and the emission of tentacles into the form of the adult without passing through any 
intermediate actinula stage. 
Properly speaking, Hydra represents a permanent actinula. Hydra (if we except the 
somewhat obscure form described by Greeff under the name of Protoliydra) may thus 
be assumed as the lowest known hydroid, and, in accordance with the Descent Theory,: 
would be the remotest ancestral form yet discovered of the Ccelenterata. 
In all cases, however, it must be borne in mind that the planula is nothing more than 
the blastodermic sac after the two leaves of the blastoderm have become differentiated. 
In some few cases it never clothes itself with cilia, and then it almost always remains, as 
long as it continues a planula, included within the gonophore ; while in the great majority 
of cases it develops cilia over its surface, and becomes free and locomotive. 
Kleinenberg, finding that in the adult Hydra the entire cellular ectoderm is composed 
of the caudate cells with an interstitial network of simple cells interposed between their 
proximal attenuated ends, while their wide distal ends form the outer surface of. the 
animal, concludes that there is here no external epithelium or epidermis. Hydra would 
thus present an apparent anomaly, inasmuch as one of the most universal features in 
ontogenesis — the development of an epidermal layer from the outer germ-lamella (ecto- 
derm) — would seem to be absent. 
This anomaly, however, is brought into agreement with the established facts of deve- 
lopment by Kleinenberg, whose observations have led him to maintain that the so-called 
egg-shell of Hydra is really a transformed epidermis, but, being needed only as a pro- 
tective investment for the embryo, is a transitory structure destined to be cast off in the 
later periods of development. 
Though this may be a correct view of the state of things in Hydra, it is certain that 
in Myriothela we have a perfectly distinct and well-developed layer which lies external 
