Norwegian Solenogastres. 65 
However attractive the supposition may be that the folds of 
Neomenia correspond to the foot and the branchiae of Chiton, this 
theory is, in sequence to the above facts, quite untenable, and no 
convincing proofs of it have been presented. Authors have riveted 
their attention on the superficial and striking similarities without 
entering into a critical judgement of the real homologies. An opi- 
nion consonant with the one advanced here has, however, also 
been expressed by THIELE (1891), who homologises the foot of Chiton 
with the whole ventral side of Proneomenia but who has been unfor- 
tunate in his homologisation in other case, as WIREN has shown. 
Subsequently (1902), however, THIELE advances a new supposition 
that the foot sole in Chiton has its homologon in the ventral furrow 
of the Solenogastres with its ciliated epithelium. 
THIELE'S opinion that the ventral fold is not a reduced Chiton 
foot has afterwards been accepted by other authors. Thus 
NIERSTRASZ (1910) defines his attitude to the problem in the fol- 
lowing words (p. 416): »Ich stelle mich ganz auf die Seite von 
THIELE und PLATE, welche von einer Verkiimmerung eines Fusses 
nichts wissen wollen. 
It may be of interest here to quote a further statement by the 
same author in connection with the question as to a presumed re- 
gressive development of the Solenogastres. He says (p. 416): 
»Meines Erachtens ist bei den Solenogastren kein einziges Organ 
in Regression; es lassen sich fiir alle Organe Entwicklungsreihen 
in progressiver Richtung nachweisen«. 
Nor do embryological facts recently brought forward ea 
1918) support the reduction theory. 
Having thus shown that the Solenogastres lack a differentiated 
foot and a peripedal or mantle cavity — in Gymnomenia and the 
above-named genera the latter is rudimentary — characters of 
fundamental importance for the Mollusca, we consequently arrive 
at the conclusion that the Solenogastres do not belong to the Mol- 
luscan phylum. 
It may, of course, be objected to this conclusion, that all 
depends on the extension that one attributes to the term » Mollusca. 
The latter has undergone many modifications in being gradually 
limited not only by excluding the Bryozoa, Brachiopoda and Tuni- 
cata formerly referred to them, but also by the addition of new 
forms. Such an additional group are indeed the Solenogastres, and 
because we have shown above that they lack the supposed molluscan 
5 
