PRIAPULUS AND HALICRYPTUS. 
205 
approach the axis of the structure. This is best understood by 
examining a longitudinal section such as fig. 3 a. The hypo- 
dermic cells forming the circumference of the spike elongate 
just as we have seen it before in Priapulus. Internally to 
these we now find large pear-shaped cells (figs. 3 a, 3 b ,p. c.), 
containing a protoplasmic network and nucleus, and tapering 
above into a fine filament which suddenly swells up again into 
a club-shaped portion. This set of cells again surrounds 
another set (figs. 3 a, 3 b, i. c .), which I have not been able to 
trace clearly, but which are probably filamentous in shape 
from base to apex and end in long hairs. The latter (/. h .) 
occupy the hollow interior of the sting. A membrane (fig. 3 
a, m.) through which the hairs project stretches across the 
opening from the lower into the upper part of the organ. A 
cross-section a little lower down (fig. 3 b) stained with chloride 
of gold, exhibits internally a cluster of small dark-stained cells 
( i . c.) surrounded by larger lighter ones ( p . c.), the latter repre- 
senting the upper parts of the pear-shaped cells (I may 
mention that in sections stained with carmine according to 
Grenadier’s prescription I have likewise obtained very instruc- 
tive views of the minute structure of these organs). The 
cuticle encircling the two groups of cells in this part is made 
up of an internal and an external dense part, staining darker 
than the rest, which lies between. Further down towards the 
base it assumes its ordinary composition again, the internal 
dark portion being wanting. Fig. 3 represents a cross-section 
through the sting, showing the hairs originating from the 
central group of cells and surrounded by the cuticular wall. 
Having considered the anatomical and histological details of 
all these sensory organs, it now remains, after having made a 
few remarks on similar organs in other animals, to show their 
relation to these. 
In Sipunculus nudus, which has recently been reinves- 
tigated by Andreae, 1 structures composed of modified hypo- 
dermic cells have been discovered, the bases of which are in 
connection with nerves (pi. xii, fig. 9). No sensory hairs 
1 Andreae, loc. cit., p. 219. 
