PEIAPULUS AND HALICRYPTUS. 
201 
cells, and he even observed the slender hairs projecting from 
the apex of the tubes which I described. This latter fact 
escaped the notice of all other observers. 
Horst’s 1 statements differ from my own in some respects, 
in so far as he holds that the interior of the spikes consists 
of irregularly branching cells of the hypodermis. The cells 
are said to converge towards their base and to have a fibrous 
structure, which gives them the appearance of radiating from 
the cutis lying underneath (see also his pi. ii, figs. 1, 2, 4, s.). 
The spikes of the body, of which we have seen that they are 
arranged in parallel rings round the body, are comparatively 
few in number. On account of the great contraction of the 
circular and longitudinal muscles of the body, these minute 
organs almost disappear within their folds or become so con- 
torted that they are of no avail for the study of their 
histological details. They can only be profitably studied with 
good immersion systems, but on account of the above-mentioned 
disadvantages I was only able to obtain one or two of them 
which were at all satisfactory. Their structure, although on 
the main points agreeing with the spikes on the proboscis, 
shows a few points of difference, and it seems to have reached 
a higher state of development. The whole organ has the form 
of a conical or somewhat cylindrical elevation. The cuticula 
(fig. 2, c.) is thick and slightly sunk in at the apex of the cone, 
leaving an aperture at the centre. The outer hypodermic 
cells become again elongated just as in the proboscis, but now 
we come to the main difference. The cells in the axis of the 
cone are enlarged at their upper part and assume a club-shaped 
form, their bases becoming apparently resolved into a net- 
work of fibres (fig. 2, c. c.). Perhaps the whole organ may be 
constructed on a similar plan to those of Halicryptus, which 
will be described below. Each of these central cells bears a 
short stiff hair ending freely into the surrounding medium, and 
as far as I could ascertain without the intervention of a special 
membrane as was the case in the proboscis. This, however, is 
nevertheless very probably present, and may have only been 
• Horst, loc. cit., p. 18. 
