1880. ] The Siphonophores. 623 
Two opposite sides of the bell walls rise as horn-shaped projections 
which embrace the axis and fit closely into certain spaces left 
between similar projections on bells of the opposite series. By a 
dovetailed arrangement of this kind rigidity is given to both 
series, and loss of energy in muscular action reduced. The action 
of the bell is as follows: Water is taken into the cavity and by 
muscular contraction of the walls is violently returned through 
_ the opening by which it entered. The resistance which the water 
thus forced out encounters from the surrounding medium, deter- 
mines the amount of motion given to the colony. The direction 
which the animal takes depends upon the angle which water pass- 
ing out of the necto-calyx makes with the line of the axis. This 
final condition depends in turn upon the position of the mouth of 
the bell as referred to the stem, and is regulated by the animal. 
When, for instance, the openings of all the bells on one side are 
at right angles to the line of the stem, and water is forced through 
them, lateral motion is given to the animal, In such a case the 
muscular contraction of the walls of the bells in a series must 
be simultaneous. Solitary action of necto-calyxes at either end 
of the series would alter the inclination of the stem in the water. 
When the bell mouths point downwards, t. e., towards the end of 
the axis opposite the float, resistance would be exerted at an 
angle less than a right angle, and as a consequence a motion in 
the direction of the axis is a result. Combinations in the action of 
different bells might be made to impart almost any motion to the 
colony. The motion in Aga/ma is seldom rapid but very graceful. 
What has been said would seem to indicate the existence of a 
nervous system, but in Aga/ma no nervous elements have yet 
been made out satisfactorily. Pigment spots found on the rim of 
the necto-calyx may be regarded as organs of sense, but that they 
are such is only probable. Agalma is sensitive to the touch of 
the finger on almost any part of the body. 
Nourishment is brought to a swimming bell by means of the 
stem, and is distributed in the bell by what are known as the- 
chymiferous tubes. These are radially situated and are four in 
number, to which may be added a tube running around the rim 
of the bell, and a medially placed vessel which unites the radial 
System with the stem cavity. 
In the earlier conditions of the necto-calyx the four radial 
tubes resemble each other very closely, and are straight vessels. 
