Norwegian Solenogastres. 63 
the circumpedal or mantle cavity in Chiton and the remaining 
Mollusca. 
A lateral sinuosity similar to that of Gymnomenia is present 
also in some other Solenogastres with a thin cuticula, and we 
have already observed it in Nematomenia. It exists, for example, 
in Heathia, and above all in Sandalomenia THIELE 1913, where it 
is the more pronounced as a row of papillae bounds it dorsaliv. 
This papillar margin strongly recalls the perinotum of a Chiton, 
and the position of the lateral nerve cords is quite homologous 
immediately beneath the respective margins. 
The said circumstances suggest a new interpretation of the 
ventral furrow of the Solenogastres and of their relation to the 
Mollusca. 
Whereas THIELE (1902) considered the ciliated furrow as a 
primitive creeping disc and the duplicature of the notum of ihe 
Chitons as »das Aquivalent des urspriinglichen K6rperrandes« of 
a discoidal ancestor (cf. 1. c. p. 304, figs. 3—5), Gymnomenia 
and the above-mentioned forms tell in favour of the opposite opinion, 
namely that this duplicature has originated secondarily from the 
real lateral side of a rounded body and has been caused by the 
development of a secondary furrow in this region. 
Gymnomenia as well as Nematomenia thus afford a possibility 
of explaining the origin of the molluscan type, and in the first place 
the Chitonean one, in a manner which yields some interesting ho- 
mologies. Its thin cuticula, as already mentioned (cf. descriptive 
part), has yielded to the traction of the septal diagonal muscles. 
The first step towards a differentiation of the body wall into a 
ventral creeping disc and a lateral peripedal furrow has thus been 
taken. In Gymnomenia a further development of the muscles in the 
Jateral sides of the rudimentary foot is observable in the crossing 
muscle fibres. If we imagine that the latter increase in strength and 
that the intestine withdraws from these lateral portions (cf. Nemato- 
menia), a rather striking resemblance to the Chiton foot will be 
presented. 
At the same time as the foot grows muscular, it loses its 
spacious lymphatic lacunae, but those at the lateral sides of the 
intestine are retained. The latter are well adapted to develop into 
the lateral vessels of Chiton. As they are in a very close proximity 
to the body surface, and as the cuticula is thinner here than at the 
