93 



subradular organ is protruded through the mouth opening. And further- 

 more that this is a normal proceeding is indicated by the fact that the 

 organ in question is provided with special retractor muscles. From 

 these observations Plate was led to say (p. 21): „Ich halte also das 

 Subradularorgan für ein im Dienste der Nahrungsaufnahme stehendes 

 Tastorgan und nehme für dasselbe diejenige Funktion in Anspruch, 

 welche sonst bei den Gastropoden von den Lippenrändern ausgeübt 

 zu werden pflegt." The following observations tend to show this to 

 be the correct view. 



While it is practicably impossible to make observations upon the 

 Chitons in their natural haunts along shore, studies may be made upon 

 them under fairly normal conditions in aquaria. For months I have 

 kept Crypt ochit on St ell er i in captivity and have frequently seen 

 the subradular organ protruded from the mouth. This invariably 

 happened with the individuals of this species after they had been 

 starved for several days and were moving about apparently in search 

 of food. It then occurred rhythmically at intervals of probably ten 

 seconds, though the rate varies somewhat according to the size of the 

 animal and other conditions. At this time the mouth is held widely 

 open which renders it possible to determine that the pushing forward 

 of the subradular organ is, so far as I have seen, invariably associated 

 with movements of the radula such as are concerned with the act of 

 biting off food and drawing it backward into the gullet. The entire 

 process will now be described. 



After its complete exposure (Fig. A) the subradular organ is with- 

 drawn into its sheath by means of special retractor muscles. As it 

 vanishes from view the forward end of the radula appears and is 

 drawn downward and backward past the mouth opening (Fig. B) and 

 seemingly penetrates far into the subradular sheath. At this time the 

 radula is almost flat, its edges being unrolled, and accordingly the 

 rows of teeth are fully exposed to view as they pass the mouth op- 

 ening. The reverse movement of the lingual ribbon now commences 

 (Fig. C), and at the same time its lateral borders are curved inward 

 toward the median line. Directly opposite the centre of the mouth 

 opening this movement brings the major lateral teeth together 

 causing their terminal cusps to accurately interlock, and in this con- 

 dition they disappear from view as the radula continues to be rolled 

 into a tube. After the last row of teeth has vanished a rounded ridge 

 appears separating the forward extremity of the radula from the sub- 

 radular organ (Fig. D). In section this particular area appears to be 

 devoid of special sense cells and its turgidity at this time is doubtless 



