Norwegian Solenogastres. 61 
cussion thereof with the following words (p. 85): »In Ubereinstim- 
mung mit allen Verfassern, die sich hieriiber åusserten, ausser 
THIELE, bin ich tberzeugt, dass die Bauchfurche der Neomeniden 
eine Mantelhöhle ist; und dass die Cloake der Chaetodermen dem 
hintersten Theil der Mantelhöhle bei den Chitonen entspricht«. 
Cogent proofs of this opinion are unobtainable, since no direct 
and indisputable transition form between Chitons and Solenogastres 
has been described. WIREN’S assumption seems, however, to be 
reasonable in view of the general habitus of the animals in question, 
as well as the fact that in Chitonellus, as PELSENEER has pointed out, 
the foot is reduced in size so as to occupy only the median line of 
the ventral side, thus somewhat recalling the ventral fold of a 
Solenogastre. If this parallel is drawn, we must compare Chi- 
tonellus with Proneomenia sluiteri, the ventral fold of which is 
relatively well developed, showing in section a rather broad shape 
and a structure of connective tissue within the epidermal covering. 
It is by the gradual reduction from such a form that we must 
explain, according to the above hypothesis, the single or the median 
fold in other Solenogastres, for instance Neomenia, where the folds 
consist of merely epithelium and very little tissue; finally the re- 
duction results in a total disappearance of the fold, as in Nema- 
tomenia banyulensis and Chaetoderma. 
In dealing with the radula and the epidermis we have, however, 
arrived at the opposite conclusion namely that it is rather Proneo- 
menia that seems to represent the final stage of a progressive devel- 
opment; hence it would be natural to find the same form as a 
final type also in the series of the fold evolution. There are indeed 
some facts which render it probable that this is the case. 
Though in Chitonellus the foot is rather reduced, it still con- 
sists, for the most part,of muscular tissue; but in no Solenogastre 
do real muscles enter into the ventral fold. It must be considered as 
extremely improbable that a reduction in size of the greatly devel- 
oped Chiton foot could have been so radical that no trace of the 
muscles has been left. Further the pedal cords would certainly, 
in One way or other, indicate such a reduction, but instead of being 
reduced they are well developed. 
If the ventral furrow of the Solenogastres corresponded to the 
much reduced peripedal groove of Chitonellus, we should find a 
corresponding arrangement of the nerve cords. Thus the lateral 
cords, which in a normal Chifon run inside the peripedal groove, 
