1864.] 
mouths of their shells. And the 
varices of shells appear to be owing 
to a cessation of reproductive en- 
largement in the animal, and hence 
the mantle accumulates its secretion 
in a thickening at the lip. 
Now, on examining a nautilus- 
shell, two large muscles are seen to 
have been placed in the lower part 
of the body-chamber, and connected 
round the involute spire by a nar- 
rower muscle ; an arrangement to 
which the shell may owe its invo- 
lute form. Beneath the muscles 
are the liver which overlaps the 
Spire, the ovaries which abut on a 
large part of the septum, and cer- 
tain digestive organs above these. 
Before any new chamber can be 
made, the shell-muscles must have 
moved forward ; and before any in- 
crease in the ovaries can take place, 
a space must be formed behind. As 
the animal steadily grows, all its 
organs would enlarge, and with each 
successive brood the distended 
ovaries would require more space. 
There is a similar gradual increase 
in the size of the air-chambers. 
And since the development of ova 
would necessitate a forward growth 
of the molluse, the discharge of the 
ovaries would leave an empty space 
behind, into which the creature 
could not retire, which would then 
be shut off by a septum moulded 
on the animal’s body; and it may 
be worth notice that at the place of 
the flaccid ovaries, both at the sides 
and outer part of the shell, the sep- 
tum extends far forward. 
The Argonaut similarly accumu- 
lates its eggs in the involute part 
of the shell, but not being attached 
to it, does not form septa. 
In the male nautilus the testes 
are placed in exactly the same posi- 
tion as the ovaries of the female, 
and, excepting the liver, are the 
largest organ in the body, It may 
therefore be concluded that the de- 
velopment of the male organs would 
produce results similar to those in 
the other sex, and likewise end in 
the formation of chambers. 
There are no other organs of the 
Notes and Correspondence. 
761 
body which are liable to periodic 
changes in size; and therefore as 
the position and progressive en- 
larcement with age of the genera- 
tive apparatus necessitate results 
like those seen in the chambers and 
septa, I regard one as the cause of 
the other. 
If this is the significance of septa 
in the nautilus, the same must be 
said of all nautiloid shells, and the 
families of Ammonites and Ortho- 
ceratites ; and as the structure of 
the phragmacone of Belemnites is 
essentially similar, it must also be 
applied to such debranchiate shells 
as arechambered. Among these is 
the Spirula, connected with a shell 
essentially like a nautilus shell. 
Professor Owen describes the greater 
part of the animal as in front of a 
shell that is enclosed by a protect- 
ing lappet on each side, which is 
regarded as anovary. Though this 
is different from the arrangement 
in the nautilus there need be no 
discordancy, for as the animal is 
not contained within, but overgrows 
its shell, so ovaries would be ex- 
pected to extend beyond the rudi- 
mentary shell; but from the defect 
of specimens, I can offer nothing 
but analogy on behalf of the homo- 
logy of the two sets of chambers. 
In the sepia the ink-bag and ova- 
ries are placed at the end of the 
animal’s body just in front of the 
mucro of the shell; and in the Be- 
lemite the ink-bag is found just 
above the phragmacone. 
Connecting the chambers is the 
tube known as the s/p/ uncle running 
through every septum to the first, 
but not through the nidamental 
capsule. In the nautilus it is often 
but partly calcified ; it changes its 
position in different genera ; and in 
some fossil-forms, as the sub-genera 
of Orthoceras, is large and compli- 
cated, being sometimes radiated 
like a coral, and perforated by an 
inner tube. 
Seeing the extreme elasticity of 
many membranes of invertebrata, 
as, for instance, the oral membrane 
of a starfish, I would also point 
