with Observations on Australian Hyclvoids. 77 



Huxleyi and A. phoenicea, and, in a still more advanced, 

 stage, in the before-mentioned section of Halicornaria. 



Structure of the Stem and Branches. 



In examining the structure of the hydrocaulus, particu- 

 larly among the Statoplea, I find that there are two distinct 

 modes of branching, one of which is mainly characteristic of 

 the monosiphonic species, while the other is only possible 

 where there is a compound stem. In the former case the 

 stem and branches are simple jointed tubes, which, when the 

 hydrophyton is large, are of considerable thickness, and the 

 branches spring from the ordinary internodes, behind or 

 between the hydrocladia, or occasionally, in some species, 

 replacing them. In most polysiphonic species, on the other 

 hand, 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 supple- 

 mentary tubes which grow up in contact with it. For 

 example, in Aglaophenia longicornis we find at the back of 

 the original slender jointed stem a stouter secondary tube, 

 and from this spring at regular intervals the alternate 

 pinnately-arranged branches. In such species as this it is 

 evident that the fascicled structure must be developed before 

 the branches can be produced, and is therefore essential to 

 the normal growth of the hydrophyton, but there are species 

 which, in the thick stem and the mode of branchinof, ao^ree 

 precisely with the typical monosiphonic forms, but which, 

 nevertheless, have the lower part of the stem and branches 

 fascicled; in such cases the adventitious tubes might be 

 absent without affectino- the s^eneral habit. 



As regards the origin of the compound stem, it is obvious 

 that the supplemental tubes are hydrorhizal elements. 

 Monosiphonic species sometimes occur, with a few irregular 

 tubes, which, springing from the hydrorhiza, have attached 

 themselves to the basal part of the stem instead of to a 

 foreign body. In Ilalicornopsis avicularis (as in many 

 Sertularians) the additional tubes are regularly present, and 

 are so numerous as to thicken the stem considerably, while 

 in the typical polysiphonic species they are, as shown above, 

 of still more importance. Keeping in mind the hydrorhizal 

 origin of the polysiphonic stem, we see that in Aglaophenia 

 longicornis, for example, every one of the main pinn^ is. 



