Norwegian Solenogastres. 57 
denticles produced by odontoblasts with rounded nuclei. The den- 
ticles are much smaller in the frontal end of the radula. In front 
the radula shows a tendency to bend laterally into a pair of pockeis, 
probably due to a division of the pre-radular pocket. In Simrothiella 
the said characters are still more marked: on each side the pockets 
have been separated from and prolonged far back outside the me- 
dian part of the radula: they have acquired a muscular sheath 
and an epithelium of the same kind as that which forms the median 
radula teeth and being a direct continuation of this epithelium. 
Hence in these diverticula also teeth are produced, which in the 
front of the radula as well as in the diverticula are turned with their 
denticles outwards (cf. descriptive section). It would be impossible 
to deduce this type from a polystichous radula, whereas, on the 
other hand, there is no difficulty in deriving it from the distichous 
type in its most primitive shape (Kruppomenia). The latter, in its 
turn, may have issued from a stage where the pharynx was lined 
with a continuous and simple cuticula by differentiation of the 
epithelium into the two kinds of tooth-forming cells, odontoblasts 
and enamel cells. 
In any case the great variation of the radula types in Soleno- 
gastres is explained in the most natural manner by assuming a 
progressive development in various directions from a primitive type 
(cf. NIERSTRASZ 1908). Of course reductions may also have taken 
place on several occasions; thus, for instance, Strophomenia may 
perhaps be developed from Anamenia by reduction of the radula, 
and in Neomenia the absence of this organ may perhaps be a 
secondary state in comparison with Gymnomenia and Wirenia 
where its absence is probably primitive. As a general rule, however, 
a progressive development has proceeded, the consummation of 
which is exhibited in the radula of Proneomenia. Also in the 
Chaetodermatina a progressive development may be regarded as 
established, terminating in Prochaetoderma. 
In corroboration of the hypothesis here put forward one more 
example may be mentioned. In Cyclomenia Nierstrasz (1902) 
described a radula which is attached to the lateral walls of the 
pharynx and not, as usual, to a muscular support on its ventral 
side. This type cannot possibly be the result of a reduction from 
a form like Proneomenia, but must have arisen from a primitive 
stage with a cuticularized pharyngeal wall (cf. NIERSTRASZ 1905). 
The absence of a radula is generally considered to be a se- 
