A NEW FAMILY OF HYDROIDEA. 133 



structure formed of the perisarc, having the shape of a cone with the apex cut off 

 (Fig. 21). The space within the latter communicates hy the narrow end with 

 the exterior, and hy the broader with the cavity of the pinna joint. Into it 

 cells of the ectoderm may enter to a slight degree, but more usually it appears 

 to be unoccupied (in spirit-preserved specimens), and I am quite unable to 

 attach any meaning to it, though it is a perfectly constant structure. It has 

 nothing to do with the reproductive structures. Possibly it may serve as a means 

 of allowing of the ingress and egress of water to and from the perisarcal tubes. 

 Any space between the ectoderm and the perisarc in the very numerous tubes which 

 compose the colony must presumably be filled by liquid. The openings leading into 

 the hydrothecse and nematophores from the stem are small and narrow, and quite 

 filled up by the soft parts. When sudden contraction takes place part of the soft 

 portions must be withdrawn through these openings and occupy space within 

 this perisarcal tube previously, presumably, occupied by fluid. If there be 

 some means of expelling this fluid then the sudden contraction of the polypes and 

 machopolypes is rendered more easy. It may be that these openings serve this 

 purpose. The openings are guarded, as it were, by two machopolypes. 



Soft Parts (Figs. 17, 18, 19, 24). 



{a) Larger branches. 



These are all polysiphonic. The coenosarcal tubes of which each is composed 

 may be divided into two divisions — (1) a central one, (2) others varying in number 

 and surrounding this. The tubes very rarely branch in such a way that two 

 running longitudinally arise from a common one, though (Figs. 24, 27, 28) they 

 frequently are united with one another by short transverse branches passing through 

 openings in the perisarcal walls. Each consists of ectoderm and endoderm 

 containing a cavity. From the outermost series arise a great number of minute 

 machopolypes or defensive zooids. The central one gives origin to all the branches 

 passing out into the pinnce and hydrocladia, with which the branches are irregularly 

 studded. As they pass from the centre to the external surface they are connected 

 with a varying number of the surrounding coenosarcal tubes, a feature which is 

 especially marked in the case of the pinna3 (Figs. 17, 24). This arrangement is 

 clearly seen when sections are cut, and has not, so far as I am aware, been noted before. 

 Bale,* in speaking of the structure of the stem and branches in the Plumulariidte, 

 says that in most polysiphonic species " the primary jointed stem is slender (the 

 requisite strength being given by the compound stem, which is only developed as the 

 zoophyte increases in size), and the branches spring, not from the jointed stem, but 

 from the supplementary tubes which grow up in contact with it. For example, in 



* Genera of the Plumulariidre, with observations on various Australian Hydroids. Proc. E. S. Victoria, 1886. 



