REPOET ON THE HYDROIDA. 
XXXV11 
it moves about as a free larva in the surrounding water. It would seem to be about 
this time that the mesosarc shows itself as a very fine structureless membrane between 
the endoderm and ectoderm. To the larva thus formed, Dalyell, by whose observations 
it was first made known, has given the name of “ planula.” 1 The planula is still a com- 
pletely closed sac. After enjoying for a time its free locomotive life it loses its cilia and 
fixes itself by one end — the aboral pole. A delicate chitinous pellicle, the foundation of 
the perisarc, is excreted over a greater or less extent of its surface ; the free or oral pole 
becomes perforated by a mouth round which a circle of tentacles has become developed. 
The larva may now be recognised as the primordial hydranth of the colony, and it only 
remains for this to become complicated by the budding of other hydranths and of the sexual 
zooids in order that it may attain the condition of the fully developed dendritic colony. 
The mode of formation of the planula now described is in its essential features that 
which has shown itself in all Gymnoblastic and Calyptoblastic Hydroids in which a 
planula has been noticed, and subjected to sufficiently careful observation, with the 
exception of Eucope polystylci, one of the Campanularians, in which Kowalevsky has 
described a somewhat different process . 2 According to this observer the ovum as the 
result of segmentation becomes converted into a blastosphere with a large central segmen- 
tation cavity, in the walls of which onty a single layer is present. From the inner 
surface of the walls, cells are now budded off, and these fill the segmentation cavity with a 
solid cellular mass which represents the endoderm. The original single layer forms the 
ectoderm, and becomes clothed with cilia. In the endodermal mass a slit-like cavity — 
the primitive gastric cavity — now shoWs itself, and thus completes the formation of the 
planula stage of the larva. 
We know as yet very little of the development of those Medusae which pass to their 
adult state directly from the egg without the intervention of a polypoid trophosome (see 
above, p. xxix). The researches of Metchnikoff have shown that in at least two of these, 
Polyxenia leuco.styla and JEgirueta fiavescens, the segmentation of the ovum results in 
the formation of a solid morula, from which a peripheral layer is differentiated by 
delamination as an ectoderm, which becomes clothed with vibratile cilia. In the central 
or endodermal cell-mass of the body thus formed the primitive gastral cavity makes its 
appearance, and soon opens externally by the formation of a mouth. On two opposite 
points the body becomes extended into a solid tentacle, and two other similar tentacles 
subsequently make their appearance in a median plane at right angles to that of the 
former, so that the symmetry, at first bilateral, becomes radial. Between the ectoderm 
and endoderm a clear gelatinous layer is excreted, and this rapidly increasing in volume 
while the larva assumes a more lenticular form, becomes the gelatinous umbrella of the 
1 Sir J. Graham Dalyell, Rare and Remarkable Animals of Scotland, London, 1847. 
2 Observations on the Development of Coelenterata, Trans. Friends Nat. Hist., Anthrop. and Ethnog. Moscow, 1873 
(Russian). 
3 Studien fiber die Entwickelung der Medusen und Siphonophoren, Zeitschr. f. wiss. Zool., Bd. xxxiv. p. 1074. 
