12 THE VOYAGE OF H.M.S, CHALLENGER. 
regard to the hydrotheca, becomes much elongated and transformed into the great pro- 
tective rib. 
The whole morphology of the corbula thus becomes beautifully distinct. We have 
only to complete the transformation by supposing the costal hydrothece, with their 
peduncle and lateral nematophores to become suppressed, and the ribs to become con- 
fluent by their edges, in order to convert the curious open cage of Lytocarpus myrio- 
phyllum, and of the Gulf Stream and Challenger Plumularians, into the ordinary closed 
Aclaophenian corbula. 
In the two species of the Gulf Stream exploration (Lytocarpus distans and Lytocarpus 
bispinosa), the hydrocladium, which is to become transformed into a phylactocarp, retains 
its normal condition for a greater distance than in Acanthocladium huxleyi continuing 
to bear from three to five scarcely altered hydrothecee before the change begins which 
results in the formation of a phylactocarp. 
An intermediate condition will be found in those instances of an open corbula (Aglao- 
phenia filicula and Aglaophenia attenuata, Pl. XI. figs. 5 and 9), in which, while the 
hydrothecee as in the ordinary closed corbula become suppressed, the leaflets remain 
distinct from one another. 
A very interesting and instructive form of phylactocarp is found in Lytocarpus 
racemiferus (Pl. XIII.). In this beautiful Plumularidan, the hydrocladia on each side for 
a certain length of the principal branches become, as in the other instances, modified 
so as to form protective supports for the gonangia (fig. 4). The modification here 
consists in the entire suppression of the hydrothecee, while the mesial and lateral nemato- 
phores are retained in a scarcely altered form. The hydrothecal internodes also continue 
distinct, and the places of the suppressed hydrothecee are taken by the gonangia, which 
are thus disposed in a single series, one on each internode, from the proximal towards 
the distal end of the rachis. Near the distal end, however, the suppressed hydro- 
theese are not replaced by gonangia, though here, on every internode, we still find the 
three nematophores, the mesial and the two lateral, of the absent hydrotheca. In this 
form of phylactocarp there are no rib-like appendages ; and the mesial nematophores, 
which in other forms become converted into ribs, here retain their normal condition. 
In Lytocarpus spectabilis (Pl. XV.) we have another instructive example of a 
phylactocarp in which no ribs are developed. Here, as in the instances already cited, 
the phylactocarps take the places of hydrocladia, of which they are obvious modifications 
(fig. 4). The proximal internode carries a hydrotheca with its normal mesial and 
lateral nematophores, but im all the other internodes the hydrothece with their mesial 
nematophores are suppressed, while the lateral nematophores are retained as a pair of 
strong blunt spines. 
In the specimen from which the figures on Pl. XV. had been drawn, no gonangia 
were developed on the phylactocarps. In another, however, a gonangium (fig. 2, p. 44) 
