132 A NEW FAMILY OF HYDROIDEA. 



The basal portion of each pinna is strengthened by the development of a very 

 strong layer of dark brow^n coloured perisarc, continuous vi^ith that of the tube walls. 

 This thick external layer ends abruptly just before reaching the distal end of either 

 the first or second joint (Fig. 26). Each joint beyond the first one or two carries 

 two pinnules. There appears to be a slight variation in the most proximal joints ; 

 sometimes the first, sometimes the first and second, differ from the rest, in bearing 

 each only one pinnule. 



The pinnules are alternate, and consist of a varying number of joints, which are 

 alternately shorter and longer, the latter only bearing the hydrothecae and 

 nematophores (Figs. 21, 26). This arrangement is constant in all specimens which 

 I have examined. In other Plumularife the pinnules of which are composed of 

 alternately longer and shorter joints, such, for example, as P. setaceoides, goldsteini, 

 delicatula* the shorter ones always bear a nematophore. This is absent in 

 P. procumbens. The hydrothecifi are cup-shaped, with a smooth margin, and are 

 placed on the side facing the central stem of the pinna. The most distal pinnules 

 carry one hydrotheca each, the proximal as many as six. The cavity is separated 

 from that of the joint by a septum pierced by a circular opening, which lies 

 near to the external wall. There is no trace of an intrathecal ridge. 



'o^ 



The nematophores are three in number on each of the larger joints ; one is 

 placed below the hydrotheca, two at the level of its upper margin. Each is 

 bithalamic with the terminal chamber cup-shaped, and the proximal one somewhat 

 canaliculate. The walls are thin (Fig. 22), except where the division into the two 

 parts is formed, at which spot they thicken considerably, and give rise to a circular 

 ridge projecting upwards into the distal chamber. The opening is single and terminal, 

 and each nematophore is only attached by its proximal end, where the walls are thin, 

 and may be thrown into slight folds. 



The walls of the pinnule joints show internal ridges, which are always more 

 prominent on the side facing the central stem of the pinna, and thin away towards 

 the opposite surface. They are always arranged thus — (1) In the larger joints 

 there is a ridge close to each extremity, with a third one corresponding in position 

 to the septum of the hydrotheca ; this varies much in development, being sometimes 

 scarcely noticeable. (2) In the smaller joints there are uniformly two ridges. (3) 

 In the projection from the joint of the pinna bearing the pinnule there is always one 

 ridge. Taken altogether the result is that each line of division in the pinnule has 

 one ridge immediately on either side of it (Fig. 21). 



In the axil of the pinnules there are present — (1) Two nematophores correspond- 

 ing in structure exactly to those described above, (2) between these a curious 



* Bale. Catalogue o£ Australian Hydroids, PI. XI. 



