The Morphology of a Solenogastre. 715 
the intervening membrane upon the outside of the radula supports. 
As fig. 15 shows this causes an unusually wide separation of the 
teeth of the opposite sides. 
Nos. 3 and 4 obviously admit a certain degree of separation of 
the supports but prevent an excessive amount of disunion. 
No. 5 acts, like muscle m (PLATE, 1897) in the chitons, to pull 
back the antero-lateral walls of the radula sac thus exposing as far 
as possible all the functional teeth. 
Obviously the contraction of the major part of muscles 7 and 
8 results in bringing the radula and its supports closer to the mouth 
opening, which as I know from personal observation is frequently 
opened widely during the act of feeding. They are definite protractors. 
While watching one individual of this species in the act of 
feeding my attention was directed to the violent rhythmical movements 
of the semicircular groove. At intervals of 5 or 10 seconds it would 
be greatly deepened, the buccal sensory plate (Mundschild) and 
mouth opening being drawn into it for a considerable distance (as 
in Fig. 19) after which they would reappear as the groove again 
became shallow. Having examined the muscular system I am now 
convinced that these movements are associated with the retraction 
of the radula; that by the contraction of muscles 9, 10, 11 and 12 
the semicircular groove is deepened, the subradular wall of the 
pharynx is pulled dorsally and somewhat backward and the front 
end of the radular supports are forced upward as in Fig. 19. The 
tongue is depressed probably by the action of some of the fibres of 
No. 7 whose contraction tends to elevate the posterior ends of the 
radula supports and certainly by the action of numerous small fibres 
radiating forward to the body wall from the front end of the 
pharynx (No. 18) and bottom of the semicircular groove. The pressure 
of the blood in all probability is an active agent. 
The action of the remaining muscles of the head region need 
not be specifically noted. Obviously they are concerned in the active 
movements of the front end of the body when in the act of burrow- 
ing, collecting food, etc. and the result of their contraction may 
be understood in a general way after an examination of Fig. 7. 
Summing up the chief movements of the radula in the 
act of feeding: the tongue is swept past the mouth opening 
by the contraction of muscles 9, 10, 11 and 12. During the 
process the tips of the teeth of opposite sides which have been 
separated by No. 2 are brought together by No. 1 and the food 
Zool. Jahrb. XXI. Abt. f. Anat. 46 
