Fewkes.] 298 [October 1, 
The tentacles do not differ from other tentacles as found among 
the Siphonophorae. They are composed of the same two layers, 
and arise from the base of the polyps. When the polyp is detached 
the tentacle never remains on the stem, but continues united to the 
base of the polyp. 
The motion of the tentacles is for the most part a simple contrac- 
tion and extension. The festoon-like position into which, according 
to Mr. Agassiz, the tentacles of Nanomia are thrown, is rarely seen 
in the tentacles of Rhizophysa. They hang listlessly down by the 
side of the stem, and, except when the animal is alarmed, are never 
moved. Their length, as compared with the distance between the 
polyps, is much greater than that represented by Gegenbaur. 
The appendages to the tentacle are the most unusual structures in 
the organization of this curious Siphonophore. Nowhere do we find 
a similar complication of structure, nor so many different forms. 
Other Siphonophorae have as complicated types of tentacular knobs, 
but no adult more than one kind, nor do they serve for more than 
one purpose. Rhizophysa has three kinds of appendages attached 
to the tentacle, and I think these have three functions; each one of 
these parts passes through a distinct and more or less characteristic 
development. 
Gegenbaur pointed out these three kinds of knobs, but did not, 
trace the first to its adult condition. The figure which he gives 
corresponds with the undeveloped stage which I have figured in my 
plate. (Plate 2, fig. 5.) This form is immature, and passes into the 
more advanced condition, fig. 6. 
Keferstein and Ehlers follow Sars in regarding the second kind of 
tentacular knob as developed from the first. That view is, I think, 
a wrong one, for the second kind of knob has a very different mode 
of development from the first, and has early formed structures which 
never appear in the former. 
There seems little doubt that Gegenbaur was right in considering 
that there are three kinds of tentacular knobs, although he studied 
only the undeveloped form of the first. 
The first and most numerous kind of tentacular knobs is found by 
hundreds along the tentacle, extending to its very extremity. As 
would be naturally expected, they are found in all stages of develop- 
ment, with the most distal the most complicated in structure. When 
fully grown, they are united to the tentacle by a secondary stem, 
which is highly contractile, and in structural character differs in no 
