185 
ment of the heads or motionless portions of the spermatozoa 
embedded in the cement. The striation of the most super- 
ficial layer invariably presents an oblique direction passing 
from left to right posteriorly, as seen in fig. 3. Focussing more 
deeply, we come upon the optical long-section already de- 
scribed (fig. 4); and then, more deeply still, we obtain the 
converse of fig. 3, the striation running from right to left 
obliquely and posteriorly. These appearances may be 
explained if we imagine a cylinder of a soft viscid material, 
to be stuck full of small bristles, each a little longer than the 
radius of the cylinder’s cross section, each placed at right 
angles to the cylinder’s surface, and passing nearly to its 
centre, in fact, arranged somewhat like the bristles of a rotatory 
hair-brush, but more deeply set. Then we must imagine the 
viscid cylinder, with its embedded bristles, to be pushed into 
a closely fitting sheath, and slowly rotated on its own axis 
from right to left, whilst it is, at the same time, undergoing 
the longitudinal movement. In this way all the bristles 
would become directed along the lines of a series of spirals, 
running from above downwards, from left to right of the 
observer, and the transverse and longitudinal sections would 
give the same appearance which we observe in the sperma- 
tophor of Tubifex. Hence we may suppose that some such 
process has taken place in the building-up of these bodies. 
I have already mentioned that considerable difference is 
exhibited in the intensity with which the spermatozoa make 
themselves apparent as a spiral fibrous structure. Some 
spermatophors are quite brown and strongly marked in this 
respect, others are much paler, and some present actually 
no trace at all of spermatozoa; so that I am inclined to 
regard them as blank “ cement-forms,” which have assumed 
their appropriate shape without enclosing any spermatozoa 
(fig. 12). 
The spermatophors of 7’ rivulorum will bear considerable 
pressure without breaking, and are of a tough leathery con- 
sistency. When they do break, they tear, along spiral lines, 
and become teazed out into fibres, as seen in fig. 11. Strong 
acetic acid facilitates this tearing, but does not otherwise 
alter them. I have not made a thorough micro-chemical 
examination of the spermatophors. They may be preserved 
excellently, apparently without any change, in glycerine 
jelly. 
; Ke already mentioned, the free vibracula of the component 
spermatozoa are sometimes to be seen moving with great 
regularity, whilst the sperm-ropes are lying within the copu- 
latory pouches. A solution of from one to two per cent. of 
