1 
THE VOYAGE OF H.M.S. CHALLENGER. 
In one of these, neither hydrothecse nor gonangia are developed, so that both 
hydranths and gonophores are always naked, or if a perisarcal pellicle is excreted over 
the body of the hydranth, as in Eudendrium vestitum (Pl. I. fig. 1), or over the 
gonophore as in Cordylophora lacustris, it is in the former case an adherent, coat from 
which the hydranth is never free as in a true hydrotheca, and in the latter case, it is 
similarly adherent to the outer surface of the gonophore, and totally different from a 
gonangium which always includes a blastostyle, from which one or more gonophores 
are budded off. The gonophores are in some cases hedrioblasts, in others planoblasts, 
and the planoblasts, with scarcely an exception, belong to the Anthomedusal section of 
Craspedote Medusae, and have the generative elements developed in the walls of the 
manubrium. 
To this “type the designation of Gymnoblastea has been given. 
In another type of Hydroid form the hydranths are protected by hydrothecae and the 
gonophores by gonangia ; and though in one family (Haleciidae) the hydrothecae may 
be so degraded as to afford scarcely any protection to the hydranth, they are still 
recognisable under this degraded form. In this type, as in the former, we meet with 
both hedrioblastic and planoblastic gonophores, and when these are in the form of 
planoblasts, they belong almost without an exception to the Leptomedusal section of 
the Craspedote Medusae, and have their generative elements developed along the course 
of the radial canals. 
The group thus characterised forms the suborder Calyptoblastea. 
Both the Gymnoblastea and the Calyptoblastea, when new hydranth buds are 
emitted by them, retain these as permanent elements of the colony. There is, however, 
another small group of Hydroids in which the hydranth buds, after attaining a certain 
degree of maturity, detach themselves and lead henceforth, like the parent animal, an 
independent and free life. This group is represented by the fresh- water Hydra. No 
perisarc is here excreted, and the generative elements are not contained in gonophores, 
but are developed directly in the deeper parts of the ectoderm of the hydranth. 
The group thus characterised forms another suborder — that of the Eleuthero- 
BLASTEA. 
In a fourth group we have again hydranths united into permanent colonies, but the 
skeletal structures which when present in the former groups are in the condition of a 
chitinous perisarc, are here represented by a hard calcareous corallum. This is permeated 
by a network of inosculating coenosarcal canals, and is overlaid by an external ectodermal 
coat. From the coenosarcal network are developed zooids of two kinds ; one (gastrozooids) 
with a mouth and stomach, and with a circlet of tentacles ; the other (dactylozooids) in 
the form of tentacula-like zooids, and destitute of mouth. The gastrozooids represent 
the hydranths of the preceding groups. Both kinds of zooids are lodged within chambers 
excavated in the corallum and lined by reflections of the superficial ectodermal layer. 
