XXVI 
THE VOYAGE OF H.M.S. CHALLENGER. 
— specially devolves, and though, as we shall presently see, the formation of the sexual 
elements is far from being in all cases dissociated from the trophosome, we may con- 
veniently designate collectively all those parts which have special charge of the protection 
and development of these elements, as the gonosome. 1 
1. The Gono'pliore. 
The most important zooid of the gonosome is the gonophore, as the hydranth is that 
of the trophosome. Its office is to give protection to the sexual elements — ova or 
spermatozoa — and to bring these to a more or less advanced stage of maturity before 
their final liberation. 
Throughout the whole of the Calyptoblastic Hydroids, and in many of the Gymno- 
blastic, the gonophores are produced as buds from the sides of a column-like body which 
springs from the ccenosarc. This body is morphologically a hydranth with its tentacles 
and mouth suppressed. It is known as the blastostyle, and in the Calyptoblastea is 
contained within a protective capsule-like chamber, which has its walls lined with 
perisarc, and is known as the gonangium. 
In the Gymnoblastea the gonophores are more usually borne as buds directly by the 
coenosarc without the intervention of a blastostyle, but whether a blastostyle be present 
or not, they are in this section never protected under cover of a gonangium. 
The gonophores may be divided into two main groups. In one of these, after it 
attains a certain degree of maturity, the gonophore becomes detached from the colony, 
leads henceforth an independent, locomotive life, and after bringing its sexual products 
to maturity discharges these into the open sea. To the gonophores belonging to this 
group the name of planoblasts (wandering buds) may be given. 
In the other group the gonophores never become detached, but discharge their 
sexual products while still forming part of the colony. The gonophores belonging to 
this group may be distinguished by the name of hedrioblasts (sedentary buds). 
Form and Structure of the Planoblast. — The planoblasts are all, with a single 
known exception, — that presented by Dicoryne , in which the planoblast is a ciliated 
tentacula-bearing sac 2 — velum-bearing Medusae (Craspedotae or Hydromedusae). 3 In form 
1 The sarcostyles do not belong exclusively to either of these systems, and we have both trophosomal and gonosomal 
sarcostyles. 
2 Gymnoblastic Hydroids, p. 292, pi. viii. 
3 The most obvious character of all Medusae consists in the presence of a gelatinous umbrella which acts as a 
swimming organ, and which carries at the centre of its lower or concave surface a more or less prominent gastral tube. 
The Medusae are divided into two primary sections — the Craspedotae (velum-bearing) or Hydromedusae, and the 
Acraspedae (without a velum) or Scyphomedusae. The Craspedotae, among which alone we find those Medusae which 
admit of being traced in the course of their development to a Hydroid trophosome, differ fundamentally from the 
Acraspedae. The most important points of difference by which the Craspedotae are separated from the Acraspedae are 
the following : — 1. The possession by the Craspedotae of a true velum which does not exist in the Acraspedae. 2. The 
absence in the Craspedotae of “ gastral filaments” which are always present .in the Acraspedae where they spring from 
