12 
Siphonophora Rosarum 
that it is practically immovable. Now stretching between this U-shaped 
rod of chitin and the transverse bar which crosses the head, there is 
a very large double muscle. Where this muscle becomes continuous 
with the transverse bar, it splits up into a number of fine tendon-like 
slips (Plate I, Fig. 6), just in the same way as the pharyngeal muscles 
do at their insertion into the anterior pharyngeal wall. From this 
it may be inferred that the muscle takes its origin from the U-shaped 
rod of chitin, and that it is inserted into the transverse bar. 
This being the case, what will happen when this muscle contracts ? 
One of two things must take place. Either the small under-lip will be 
drawn up, or the transverse bar will be pulled down. The former is 
scarcely likely to happen, because the muscle does not lie in the right 
direction to produce this effect; but more than this, the origin of the 
muscle is in this under-lip. The only other alternative then is that the 
transverse bar will be pulled down. The idea that it is the transverse 
bar which is moved, is supported by the fact that, situated on either 
side of the middle point of the bar, is a muscle stretching between the 
bar and the roof of the head (Plate I, Fig. 5), and the only function 
which these muscles can have, is to restore the bar to its original 
position after some displacement. It is true that these muscles are 
small compared with the displacing muscle, but, as will be seen later, 
they are probably cpiite sti’ong enough to perform the work they have 
to do. 
Having now described the arrangement and function of the various 
muscles, we will return to the original problem of how the setae are 
controlled. First, the strong muscle inserted in the centre of the 
transverse bar contracts, pulling the bar downwards. The bar in its 
course will carry down with it the A-shaped rods attached to its centre 
(see Plate I, Fig. 1), and these in their turn will again bring pressure to 
bear upon the lower jointed parts (j in Plate I, Fig. 1) of the A. Now, 
as has been described before, the rods of chitin attached to this lower 
part are continuous with the bases of the setae, and my suggestion is 
that the upper parts pressing on the lower parts of the A-shaped rods 
cause, by the bending inwards of the lower parts, the chitinous rods to 
take up a lower position in the head, and at the same time carry the 
bases of the setae along with them ; the total effect being, that the setae 
are protruded more or less vigorously, depending upon the force with 
which the muscle ( l.pr.m .) contracts. 
The fact that the posterior pair of setae have a small protractor 
muscle, does not interfere with the general working of the apparatus, 
