A NEW FAMILY OF HYDROIDEA. 131 



Under the lens the tubes making up the branches can be clearly seen, the 

 surface being marked by darkish brown lines, due to the edges of the perisarcal 

 tubes seen in optical section (Fig. 25). The tubes on the exterior run parallel 

 to one another along the length of the stem, branching very rarely. From the 

 surface arise irregularly, and on all sides — (1) large pinnate shoots, the proximal 

 parts of which are covered with one or more layers of dark-brown perisarc, within 

 which only one tubular cavity is contained; (2) smaller branches (hydrocladia) , 

 corresponding to the pinnules in structure ; (3) a great number of nematophores, 

 or protective cases, for the minute defensive zooids which are characteristic of the 

 group. 



In transverse section (Figs. 27, 28) the branch is seen to be made up of a great 

 number of tubes — as many as 40-60 in a branch of average size — the number varying 

 with the size of the branch. 



Each tube has a definite and thick perisarcal wall, which shows clear indications 

 of arrangement in layers. In places the cavities within the tubes are seen to be (Figs. 

 24, 27, 28) in communication with one another. A striking feature of both trans- 

 verse and longitudinal sections is the presence of a distinct central tube, always clearly 

 recognisable, both on account of its size and the slightly yellow colour of its walls 

 when compared with the surrounding ones (Fig. 24). This central tube, which, from its 

 relations to the other parts, is probably to be regarded as homologous with the main 

 stem of a monosiphonic form, passes along all the branches of the colony, and from 

 it arise every one of the pinnae and hydrocladia, though, as will be shown shortly, 

 these may be also connected with the other tubes which go to form the polysiphonic 

 stem. Professor Allman has figured* in Aglaophenia coarctata a connection existing 

 between the various tubes of a polysiphonic stem, and this is most clearly seen in the 

 species under consideration. The connection is, however, somewhat different from 

 that obtaining in A. coarctata, which is thus described by Allman : — " Communication 

 is effected by very short processes, which are given off from the component tubes, 

 those of two juxtaposed tribes meeting one another and inosculating in such away 

 as to suggest the conjugation of a zygnema." In longitudinal section (Fig. 24) the 

 various tubes are seen to be arranged so that they run parallel to the central one, 

 and at intervals their walls are pierced by apertures. Where the pinnfe and 

 hydrocladia pass through, the walls of the latter are connected with those of the 

 stem-tubes, and apertures are formed opening into the cavities of the latter. 



(b) Structure of the pinnae, etc. (Figs. 21, 22, 23, 25, 26). 



The origin of these from the stem is shown in figure 25, their structure in 

 figures 21, 22, and 26. 



* "Challenger" Eeports, Hydroidea, Part I., Plumulariidip. 



