Norwegian Solenogastres. 15 
Chitons which undoubtedly speak in favour of this supposition (ci. 
NIERSTRASZ 1910). 
From the above arguments, it is evident that we have to com- 
prehend the Solenogastres, both Chaetodermatina and Neomeniina, 
as organisms with a more primitive organization than Chitonidae 
and showing affinities to different groups of worms. Their systemati- 
cal position is among these animals, as previous authors have sup- 
posed and THIELE (1902) has established, though his opinion has not 
yet been generally accepted. In lacking a foot, a mantle and a mantle 
cavity with molluscan ctenidia, organs of essential importance for the 
characterisation of the molluscan phylum, the Solenogastres must be 
excluded from the Mollusca and thereby also from the Amphineura, 
though they show an undoubted affinity to the Chitons. Their 
systematical position has points of junction in different di- 
rections: they combine characters of Plathelminthes with those of 
Nemertines, Nematods and Archiannelids, but they are higher or 
else differently specialized, above all in the possession of a con- 
tractile heart. 
The origin of a heart is certainly a primitive process in the 
Solenogastres and may be explained by the following argumentation. 
As the circulatory organs are intimately correlated to the re- 
spiratory ones, the heart, when originating, comes to lie in close 
vicinity to the part of the body where respiration takes place. In 
the Solenogastres and the Mollusca this function, as a consequence 
oi the cuticularization of the body and the differentiated structure 
of its cutis, has been carried from the body surface where it has 
passed on primarily, to the points where cuticula is absent and 
the cutis is thinnest, that is in the cloaca (Solenogastres) and the 
mantle cavity (Mollusca). Thus it is quite natural that in the 
Solenogastres the heart assumes a position in the posterior part of 
the body, and that in the cloaca different kinds of respiratory organs 
tend to arise. The origin of a heart requires an organization re- 
sembling that of a Nemertine in the respect that distinct blood 
vessels are developed in the mesenchyma. When a contractile heart 
begins to differentiate the first step is a concentration of the mesen- 
chyma at the most suitable point. This concentration gives rise to 
a cavity (chiefly on the under side of the heart where most oi 
formative substance is present) and thus originates the pericardium. 
If there is sufficient mesenchyma in the surroundings of the 
increasing heart, the pericardium may remain a closed cavity of its 
