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brushlike wreath of long and pointed thorn- 
like spicules. 
The mouth opening is flanked by the paired 
triangular buccal plate, and the entire front 
end can be withdrawn into the semicircular 
groove. The subepidermal muscle layers, with 
the exception of those in the tail region, are 
very weak, and the diagonal one is almost 
totally lacking. Three pairs of distinct retrac- 
tors in the front end, and six pairs of gill re- 
tractors, pass laterally from the pericardium 
and are present with the thick radula muscles. 
The nervous system consists of paired brain 
lobes, the two halves connected by a strong 
commissure and of three pairs of precerebral 
ganglia. The buccal connective leaves the brain 
separately, whereas the lateral and ventral ones 
have a common root. The buccal system con- 
sists of three ventral commissures and a single 
dorsal one; an unpaired subradular ganglion is 
present. 
The digestive tract begins with a wide phar- 
FlG. 6. Prochaetoderma calif ornicum. A transverse 
row of the distichous radula, showing basal plate, 
tooth, and lateral plate. 
FIG. 7. Prochaetoderma calif ornicum. Differ- 
ent kinds of spines from body regions a, b, c, d, 
illustrated in Fig. 9. 
ynx, its epithelium bearing a distinct cuticle 
which, in two large lateral pockets, is differen- 
tiated into a pair of mandibles. Small salivary 
glands are developed at the junction of the 
pharynx and the short ciliated oesophagus. A 
glandular subradular organ lies in front of the 
large distichous radula; it consists of 9-12 
transverse rows of teeth (Fig. 6) and is situ- 
ated in a distinct radular sack. The oesophagus 
connects with a short wide stomach from which 
the long rectum and the short sacculate liver 
originate. 
