16 Nils Hj. Odhner. 
Externally, the whole radula apparatus is covered with a 
thick muscular sheath (fig. 4), which appears in front of the 
oesophagus as a semi-circular band continued chiefly on its under 
and lateral sides into the muscular sheaths of the lateral diverti- 
cula. On the upper side of the radula the band in question 
sends fascicles backwards at both sides of the oesophagus towards 
the median part of the radula sac, which they encircle: also the 
salivary glands are covered by a thin muscular layer. Only in 
their foremost half are the lateral and median portions combined 
with each other by means of muscular cords; at the rear they 
are separated. From a point just behind the salivary glands two 
muscle cords detach from the sheath, running backwards and 
fixing the radula to the sides of the body. 
It is a remarkable fact that it is within the lateral divertic- 
les that Simrothiella presents the form of teeth characteristic of 
other forms of distichous radulae in the Solenogastres. From 
this fact it would appear as though these diverticles represented 
the real radula sac which had been secondarily cloven and that 
the median portion is a new acquisition. But what militates 
against this view is the reversity of the teeth in comparison 
with other Solenogastres: the denticles are not turned towards 
the median line, but laterally. The case can only be explained 
if we assume that already at the first stage of development of 
the radula a flexion to the sides took place, so that the primary 
epithelium assumed a reversed position in the lateral diverticles, 
the embryonic epithelium and the primary tooth shape remaining 
unchanged. Though it might be correct to homologise the end 
of the median portion with the radula sac in the Chitons and 
the Gastropoda, it is nevertheless impossible to assume a similar 
growth of the organ as in those classes of Mollusca. As, evi- 
dently, both the lateral and the median ends serve as a centre 
of origin, the growth of the radula proceeds in these three regi- 
ons simultaneously, and thus it must be assumed that the fron- 
tal part thereof (or the middle of the continuous tooth series), 
where the teeth are most differentiated, is the oldest portion, and 
that new elements, as the animal increases, are added by repeated 
apposition to the ends of the radula. 
From the facts just mentioned it follows that the growth of 
this radula is different from what is known in the Gastropoda 
‘where an incessant moving forward of the radula takes place out 
