Norwegian Solenogastres. 55 
tions of the Solenogastres (1892), represents the opposite opinion 
to which he was led by the fact that the radula in the Solenogastres 
exhibits very extreme types of development ranging from a very 
complete structure to entire absence. WIREN has, however, pointed 
out that the radula in this group of animals, in view of its cuticular 
nature, represents a different stage of development than in the 
higher Mollusca where the radula is losely incumbent and steadily 
progressive on its substratum. This circumstance renders the re- 
duction theory very improbably, for it must be admitted that a 
reduction from the higher type of radula, which in all cases where 
this organ is retained has proceeded to an equal stage of fixed 
cuticula, assumes a very remarkable coincidence of conditions. It 
is more natural to suppose that the state of a cuticular development 
is primitive and that the different types are specializations in dii- 
ferent directions. Such an opinion has also been expressed bv 
NIERSTRASZ (1908), who summarizes his results upon the relation 
of the radula forms in the Solenogastres in the following words 
(p. 27): »Wohl steht fest, dass eine Herleitung vom gleichen Appa- 
rate der Chitonen nicht erlaubt ist, sondern dass die Entwicklung 
der Solenogastrenradula eigene Wege eingeschlagen hat. Die 
Radula der Solenogastren entwickelt sich nun meiner Meinung 
nach in verschiedenen Richtungen, welche zu den höchstentwickel- 
ten Typen wie wir diese bei den Proneomenien und Dinomenia 
finden, fuhren«. 
In sequence of the above arguments, I consider the primary 
state of the evolution to be, as a rule, that in which no radula has 
yet been formed or only a very imperfect one exists, whereas, on 
the other hand, a well developed and mighty one is a secondary 
acquisition. The development of the radula in the Solenogastres, 
in other words, has proceeded in a progressive direction, a view 
at variance with the opinion which is still, though not generally, held. 
The question as to what form of radula is to be considered 
to be the most primitive has been variously answered. While THIELE 
(1894, 1902), holds it to be the polystichous type, PRUVOT 
(1902) maintains a contrary view. He calls attention to the fact 
that »la dépression pharyngienne ou se forme la radula, et la 
radula elle-méme sont en rapport étroit avec la portion terminale 
des conduits salivaires, que la radula est distique quand les deux 
conduits débouchent isolément, et devient polystique quand ils 
aboutissent å un orifice commun; d’ot cette conclusion que la 
