1883. ] The Siphonophores. 835 
although their outward resemblances are so close. A somatocyst 
and bell cavities exist in both. The radial chymiferous tubes 
have the same inequality in length and a like tortuous course 
brought about by the unequal development of the bell walls. 
One peculiarity of the radial tubes in the bell of Galeolaria is that 
two of the four radial vessels are connected by a smaller vessel 
or lateral branch which extends through the bell walls parallel 
with its rim. The posterior swimming-bell of Galeolaria likewise 
differs in shape from that of Diphyes. One of the most interest- 
ing of the differences in general form is the existence in Galeolaria 
of two circular gelatinous plates which extend backward from the 
lower side of the posterior bell rim, one on each side of the me- 
dian line. These small disks have an important function to per- 
form in the movements of the medusa, for they serve as rudders 
by which the direction which the water takes on leaving the bell 
cavity is determined. The steering of the animal while it is in 
motion is thought by some to be brought about in the following 
manner: When the lower bell by a simultaneous contraction of 
its walls on all sides drives the water violently from its cavity 
through the bell opening, the liquid thus expelled strikes the sur- 
rounding medium and meets with a resistance. The result is 
that the animal itself is driven forward. The direction which the 
water takes as it leaves the bell depends upon the angle at which 
these disks are set on the bell margin, and by altering this angle 
the direction in which the animal moves is determined. Both an- 
terior and posterior bells in Galeolaria contribute to the onward 
motion, although propulsion is brought about in the main by the 
posterior. 
In the genus next to be noticed, allied in many respects to 
those already studied, the disproportion in size of the two 
swimming-bells is very apparent on account of the anterior bell 
being so very much reduced in size. As it is much smaller 
than the posterior it performs only a very small amount of work 
in the onward motion of the medusa. A genus which possesses 
these characters is called Abyla 
Abyla is smaller than Galeolaria and somewhat larger than 
Diphyes. It is more sluggish in its movements than either, and 
consequently more easily captured. 
1 Abyla is very common in the Mediterranean, but has not yet been taken on our 
coasts, 
