192 Prof. Carl Claus on the 
new special assumption appears arbitrary and unfounded. 
From the two axioms follows the division of the Siphono- 
phora into two primary divisions, which Hiickel denominates 
Siphonanthe and Disconanthe, and which, according to their 
origin, would be referred, the former to the Anthomeduse 
and the latter to the Trachymeduse. The inadmissibility of 
this diphyletic derivation has already been shown by another 
hand, and the contradictions have been indicated which would 
result for the structure and development of the Velelle from 
the association with octoradial Meduse*. It is not only that 
the stage of the radiate Disconula only follows upon a simply 
constructed bilateral stage of development, rendering 1% pro- 
bable that here this is preceded by a bilateral division like 
that in Siphonanthan larve, but also the mode of origin of 
the mantle, which is by no means to be referred directly to 
the Medusan umbrella, as well as the development of an 
abundant vascular net and powerful muscular layer on the 
aboral surface, in contrast to the non-vascular and non-mus- 
cular exumbrella of the Meduse, cannot be reconciled with 
Haeckel’s views. 
Against the Medusa-theory, however, in whatever form or 
modification it may be put forward, I have in my former 
paper urged another argument, which has been entirely ignored 
by Haeckel. I remarked that ‘ another consideration renders 
it improbable that the sexual form of the Hydroid polyps in 
its perfect form furnished the starting-point for the production 
of the Siphonophora, seeing that its ontogenetic origin is pre- 
luded by Hydroid-stocks, which consequently, even tn a Me- 
dusat altered by dislocation of particular parts of the body and 
transformed into the stem-form of the Siphonophora, must have 
recurred in the development of the latter.” “‘ The direct develop- 
ment (without alternation of generations) of individual Hydrowd 
Meduse { ts, however, unquestionably only a subsequent secon- 
dary condensation of the developmental process, which, there- 
fore, we are not justified in taking as the starting-point of the 
derivation.’ The Medusa-theory, however, commences with 
this subsequent, secondary, hypogenetic development of the 
stem-form, which is already repeated as a Medusa in the 
bilateral (Siphonula) or radial (Disconula) Siphonophoran 
larva, and consequently leaves the older and originally meta- 
* Chun, Sitzungsb. der k. preuss. Akad. der Wiss. Berlin, 1888, 
Bd. xliv. pp. 3,4. See ‘ Annals,’ ser. 6, vol. iii. pp. 216-218, 
+ As supposed by Metschnikoff and also by Haeckel in his “ Proto- 
meda.” 
{ To these belong the Trachymedusce and also, therefore, Haeckel’s 
“ Archimeda,” 
