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THE VOYAGE OF H.M.S. CHALLENGER. 
an open orifice which is usually very oblique. In some very rare cases (Pl. VIII. figs. 
4—7, and Pl. XIX. figs. 1-3) the lateral nematophores are absent. 
In a singular and beautiful species (Acanthocladium hualeyi, Pl. 1X.) contained 
in the Challenger collection, the hydrocladia become towards the dorsal extremities of the 
branches atrophied, lose their hydrothece, and are converted into spine-like appendages, 
which carry from base to apex two longitudinal alternate series of minute cup-shaped 
nematophores (Pl. XX. fig. 3). 
b. Cauline nenatophores.—These are usually small, cup-shaped bodies, situated on 
various parts of the hydrocaulus, with the cavity of which they communicate. They are 
generally disposed in linear series (Pl. XVII.) on the main stem and principal branches, 
and often form definite symmetrical groups of two at the points where the hydrocladia 
spring from the stem. Though quite different from the moveable nematophores of the 
Eleutheroplea,—being seated on a wide base which fixes them to the stem,—they would 
seem to be in many cases easily detached. 
c. Gonosomal nematophores.—The system of nematophores attains in the gonosome 
of the Phylactocarpal Statoplea a remarkable development. The characteristic denticles 
which are found on the margins of the leaflets, which combine to form the walls of the 
corbula (Pls, XI. and XII.) in Aglaophenia, are only slightly modified nematophores. 
These denticles are tubular with a terminal orifice, and their cavity is in communication 
with that of the leaflet, while this opens into the common basal rachis of the corbula. 
In most cases a nematophore is specially developed at the base of each leaflet in the form 
of a spur. In one remarkable form (Pl. XX. fig. 6) branching chitinous ridges are 
developed on the walls of the corbula, and enclose cup-shaped nematophores in bifurcations 
of their branches. 
But nematophores are largely developed not only in the corbula, but in the other 
forms of Phylactocarp, where they constitute a very characteristic feature. They occur in 
such cases along the branches of the Phylactocarp, or along the common basal rachis, and 
are usually present in the form of denticles or cup-shaped bodies, arranged in two opposite 
or alternate series. In Lytocarpus spectabilis (Pl. XV.) they are in the form of a pair 
of stout opposite spines on every internode of the phylactocarpal rachis, and are each 
provided with a terminal aperture and also with a lateral aperture close to the base 
(fig. 5). In Lytocarpus racemifera (Pl. XIL) there is, in addition to the par of 
opposite spine-like nematophores on each internode, an azygous spine on the front of 
the internode (figs. 4-6). A similar azygous spine-like nematophore is in Lyto- 
carpus spectabils carried by that internode of the rachis which supports a gonangium 
(fig. 2, p. 44). 
