844 THE AMERICAN NATURALIST. [Vor. XXXIV. 
they must represent these organs in an extremely simple form, 
in a form, moreover, which strongly suggests their origin from 
invaginated appendages serially homologous with those of the 
cephalic and thoracic segments. Whether these delicate 
organs may be everted by blood pressure and withdrawn by 
muscular action, like the collophoral tubes of the Collembola 
and the eversible sacs of Thysanura (Machilis, ¢.g.), remains to 
be determined. If this is the case, the hedge of bristles in 
front of the lung-book flap would seem to have a definite 
protective function. 
Another lacuna in the observations of Hansen and Sórensen 
refers to the caudal flagellum. According to Grassi this organ 
contains altogether thirteen or fourteen joints. The greatest 
number of joints present in any of Hansen and Sórensen's 
specimens was nine, and on Grassi's authority they have added 
five joints of the same structure in fainter outline to the flagel- 
lum in their Fig. 1, Taf. IV. The flagellum is very easily 
broken, even after the animals have been transferred entire to 
alcohol, so that in upwards of a hundred specimens I found the 
structure complete in only ten individuals. Eight of these had 
fifteen, one had fourteen, and one had only eleven joints in 
the flagellum. There could be no doubt that in all these cases 
the flagellum was complete, as the terminal joint ends in a 
point and is longer and of a different shape than the preceding 
joints (Fig. 1). In my specimens, joints one to eleven have 
the structure described by Hansen and Sórensen.! Each “is 
fusiform, as if composed of two truncate cones, one long, the 
other short, united at their base, and adorned with two rings 
of backward-turning setiform hairs. In the foremost ring, situ- 
ated on the widest part of it, we find, where we have been 
able to count the number, eight long, slightly curved hairs; in 
the hindmost terminal ring, sixteen much shorter, thinner, and 
less curved ones.” In my specimens the posterior circlet of 
appressed bristles is lacking on the twelfth, thirteenth, and 
fourteenth joints. The terminal joint is twice as long as any 
of the preceding, fusiform in shape, and provided with two 
circlets of long, curved bristles. 
Va. cit; p. 233. 
