12 THE VOYAGE OF H.M.S. CHALLENGER. 



regard to the liydrotlieca, becomes mucli elongated and transformed into the great pro- 

 tective rib. 



The whole morphology of the corbula thus becomes beautifully distinct. AYe have 

 only to complete the transformation by .supposing the costal hydrothecse, with theli- 

 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 Lijtocarpus myrlo- 

 phyllum, and of the Gulf Stream and Challenger Plumularians, into the ordinary closed 

 Aglaophenian corbula. 



In the two species of the Gulf Stream exploration [Lytocarpiis distans and Lytocaiyus 

 hispinosa), the hydrocladium, which is to become transformed into a phjdactocarp, retains 

 its normal condition for a greater distance than in Acanthodadiiim huxleyi continuing 

 to bear from three to five scarcely altered hydrothecse 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 (Aylao- 

 phenia Jilicula and Aglaoplienia attenuata, PI. XL figs. 5 and 9), in which, while the 

 hydrothecae as in the ordinaiy closed corbula become suppressed, the leaflets remain 

 distinct from one another. 



A very interesting and instructive form of phylactocarp is found in Lytocarjni.i 

 racemiferus (PI. XIIL). 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 mocbfication here 

 consists in the entire sujipression of the hydrothecse, 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 hydi-othecse 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- 

 thecse 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-Hke appendages ; and the mesial nematophores, 

 which in other forms become converted into ribs, here retain their normal condition. 



In Lytocarpus spectahilis (PI. XY.) 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 ob-sdous modifications 

 (fig. 4). The proximal internode carries a hydi-otheca with its normal mesial and 

 lateral nematophores, but in all the other internodes the hydi'othecse 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 XY. had been drawni, no gonangia 

 were developed on the phylactocarps. In another, however, a gonangium (fig. 2, p. 44) 



