134 A NEW FAMILY OF HYDROIDEA. 



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.* Keeping in mind the 



hydrorhizal origin of the polysiphonic stem we see that in Aglaophenia longicornis, for 

 example, every one of the main pinnae is equivalent to a separate shoot of such 

 species as A. parvida, a fact which is further illustrated by the presence, near the 

 base of the stem in the latter species (and, indeed, in many others), of a long 

 oblique joint similar to that which exists near the base of each pinna in A . longicornis. 



/ have not hitherto met with any species with branches springing 



both from the jointed stem and the added tubes."* 



It will be seen at once that an important difference exists in this respect between 

 the species now described, and those examined by Mr. Bale, and, so far as I am 

 aware, any other investigator. Not only is there a very distinct connection between 

 the soft and hard parts of all the tubes of the polysiphonic stem, but the central 

 one, though distinguishable from the rest, has the same fundamental structiire as 

 the latter so far as its walls are concerned, and shows no traces of joints. f The 

 question naturally arises What elements are we to consider as entering into the 

 structure of the polysiphonic stem in this form ? 



We may regard it as composed of 



(1.) A central tube equivalent to the hydrocaulus of a monosiphonic form, 

 which is surrounded by a series of added but modified hydrorhizal 

 elements, or 



(2.) A number of hydrorhizal elements forming branch-like structures, and 

 giving off pinna?, or 



(3.) A number of hydrocauli in close apposition to one another. 



The last is the most improbable, since, if it were the case, we might expect pinnse 

 to be given off from all or any of the tubes composing the stern, whilst they all arise 

 primarily from the central one. The same objection applies to the second, and the 

 distinction which undoubtedly obtains between the central and all the other tubes 

 seems to point to the fact that the former is the fundamental portion, the latter being 

 secondary structures. At the same time a hard and fast line of distinction between 

 hydrocaulus and hydrorhiza cannot possibly be drawn. In some forms of 

 Plumulariidse we find the pinnae arising directly from the hydrorhiza, in others they 

 arise from the sides of a hydrocauline structure which grows upwards from the 



* The italics are mine. 



t At the free extremity of each branch it is continued directly on into a jointed stem forming the centre of a pinna, 

 and may probably be regarded as having lost its jointed nature subsequent to its being completely enclosed by the 

 surrounding tubes of the polysiphonic stem. 



