622 The Siphohophores. [ September, 
be seen in Agalma, but is.not so well marked as in certain allied 
genera. Another suggestion for the homology of the float of 
Agalma is that it is a bud the same as certain of the other struc- 
tures along the axis. This view was first published by Metschni- 
koff. I shall consider it at length when I speak of the develop- 
ment of this part. 
The air sac in Aga/ma contains air or gas, and it opens into the 
cavity of the stem by an aperture opposite that into the surround- 
ing water. The opening from the float into the water is sur- 
rounded by a sphincter muscle and dark crimson pigment spots 
of unknown function. , 
The Necto-calyxes.—The individual which performs the func- 
tion of moving the colony through the water is called the necto- 
calyx or swimming bell. These are found occupying about one- 
third of the whole axis of the animal, and are arranged in two 
rows. They are transparent, bell-shaped and easily detached. 
Each row contains from ten to fifteen members. All the buds 
which later develop into necto-calyxes are grouped together in a 
botryoidal cluster just below the float. No necto-calyxes are 
developed from any part of the stem except this cluster of imma- 
| ture buds just under the 
float. The growth of an 
adult necto-calyx froma 
bud is very complicated, 
and there is no uniform- 
ity of opinion among 
naturalists as to its 
method. 
A necto-calyx is sim- 
ply the locomotive part 
of an ordinary hydroid 
Medusa. It is the bell, 
and the proboscis and 
tentacles are wanting, a5 
would naturally be ex- 
pected in consideration 
of their function. i 
Each necto-calyx is united to the stem at a point diametrically 
opposite the entrance into the cavity of the bell, and the approxi- 
mating sides of consecutive necto-calyxes fit closely together. 
Fic. 2.—Necto-calyx. 
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