A NEW FAMILY OF HYDROIDEA. 135 



hydrorhiza. Sometimes the structure l)earing the pinnae, or the central stem of the 

 pinnaj itseh', may be strengthened by the addition of tubes clearly growing up from 

 the hydrorhiza. These tubes, in some monosiphonic forms, may be feebly developed, 

 whilst in typical polysiphonic ones apparently homologous structures may be constant 

 and greatly developed. In the former case the distinction between hydrocaulus and 

 hydrorhiza is clear enough, but then it the true polysiphonic form be investigated, we 

 find that the supplementary structures grow around the primary jointed stem, and (1) 

 finally (as in Aglaophenia longicornis) give rise to the pinnate branches, just as does the 

 hydrocauline tube in such a form s,fi P.falcata, the jointed stem bearing no branches; 

 or (2) they form (as in P. procnmbens) an enclosing mass for the original tube which 

 alone gives off the pinnae, though these may be connected with the hard and soft parts 

 of the enclosing branches. Thus what must be regarded as homologous structures, 

 may either (1) be feebly developed, and retain their original hydrorhizal nature ; or (2) 

 be strongly developed and (a) give rise to pinnae, and, as it were, usurp the function of 

 the original hydrocauline tube which they support, or (b) assume an intermediate form, 

 the original tube which they inclose alone giving rise to pinnae, with which, however, 

 as well as with the former they are in organic connection. 



[b) Pinna, etc. 



There is little to say with regard to the structure of the soft parts contained in 

 the pinnae ; the hydranths have the form typical of the genus Plumularia, with the 

 body divided into two parts by a central constriction, the distal part bearing a single 

 circlet of solid tentacles at the base of a broad hypostome, the proximal half being 

 somewhat globular and presenting no special feature. 



The machopolypes of the main branches and the pinnae are of precisely similar 

 structure, corresponding to those of other forms which Lendenfeld has distinguished 

 as "guard animals with corticating capsules."* There is no trace of any with 

 adhesive cells such as are found in the genus Aglaophenia. 



Each machopolype consists of a proximal tubular part lying in the proximal 

 half of the nematophore and a distal swollen part, which contains rounded 

 nematocysts. I have been unable to study the living form, and have only seen 

 spirit specimens, in which the soft parts are of course much contracted ; in these, 

 lines running from the head down the stalk probably indicate muscle fibres, the 

 machopolype being capable of great extension. 



In figure 17 is represented a restoration of the soft parts of a small portion of 

 one of the polysiphonic stem, together with one of the hydrocladia and piunai which 

 arise from it. 



♦ Zeitsch. t. Wissen. Zool., Vol. XXXVIII., p. 335. 



