vi THE VOYAGE OF H.M.S. CHALLENGER. 
the whole of the remaining representatives of the order present it in a more or less 
modified condition, while they have in almost every instance become variously compli- 
cated by the formation of hollow buds, whose cavity is continuous with that of the 
primary Hydroid, with which most of them remain in permanent union. Many of these 
buds become elongated into branches, and thus give rise to complicated colonies, which 
often present luxuriant tree-like growths, and repeat with wonderful fidelity the most 
elaborate ramification of the vegetable kingdom (Pis. II., IV., IX., &c.). 
Even in Hydra, however, this complication by budding, and the consequent formation 
of dendritic eolonies, is foreshadowed by the emission of buds, but here the buds, after 
attaining maturity, detach themselves, and leave the parent simple as before. 
It will be hereafter shown (page xxviii) that even the generative buds which have 
become detached under the form of Medusae or “jelly-fish,” in order to spend henceforth 
a free life in the open sea, may be referred to the same fundamental type of form. It 
must accordingly be borne in mind that not only the polyp-form as shown in Hydra, 
but the Medusa-form as shown in the Craspedotae or Hydro-Medusae (see below, page xxv, 
note), must be included under the common order, Hydroida. 
II. Disposition of the Fundamental Layers. 
The parts which enter into the composition of the Hydroid body are disposed in 
two fundamental cellular layers, separated from one another by a thin, structureless, 
excreted membrane, while the whole is for a greater or less extent, in the great majority 
of cases, surrounded by another structureless membrane, also the produet of excretion. 
Of the two cellular layers the more internal one lines the body-cavity and its offsets, 
and forms the endoderm ; the more external, which is exactly coextensive with this, is 
the ectoderm, while the thin, structureless membrane which everywhere intervenes 
between endoderm and ectoderm is the mesosarc, 1 and the structureless coat which in 
almost every case surrounds and protects the soft parts in the stems and branches is 
the perisarc. 
A Hydroid colony, then, such as we meet with in the great majority of instances, 
presents a ramified growth whose stems and branches are elongated, intercommunicating 
tubes, the common cavity of which forms the nutritive cavity of the colony, and whose 
walls are composed of endoderm and ectoderm, separated from one another by the 
mesosarc, and protected externally by the investing perisarc. The living nutritive tube 
which lies within the perisarc and is the common basis from which all the buds are 
emitted is known as the ccenosarc. 
1 This is the “ Stiitzlamelle ” of Reichert, who was the first to call attention to it. It must not be confounded 
with the “ mesoderm ” or middle germinal layer of the embryo, a layer which does not occur in the embryonal develop- 
ment of the Hydroida. 
