XAUTILID^. 
Ill 
fixity of the body of Nautilus during the inhalation and forcible 
ejection of the respii'atory ciu’reiits is efl:ected by the shell-muscles 
reacting upon one another, on the principle of a spring-purchase, 
rather than by simple tractioji, as illustrated by the withdrawal of 
a Gasteropod within its retreat, or the closure of a Conchifer by the 
adductor muscles. 
“ This view, which is supported by the foregoing facts, has its 
principal basis in the line of direction of the shell-muscles, and the 
angle at which they meet one another, at the root of the funnel- 
lobe ; for, the outer extremity of each being fixed, it follows that 
the first effect of the contraction of the muscular fibres would be to 
increase the angle just noticed ; and this cannot possibly be accom- 
plished, according to the recognized laws of muscular action, with- 
out tending to throw apart the points of origin, or, in other words, 
exerting outward pressure against the internal wall of the shell, 
and thus, as it were, jamming the occupant tightly in its cell.” 
In order that the above description may be more readily compre- 
hended, a reduced copy of the figure given by Sir Eichard Owen in 
his ‘ Memoir on the Pearly Nautilus ’ (1832, plate iii. fig. 2) is here 
appended. 
Under surface of the head of Nautilus pomijilius, with the mantle divided and 
the funnel turned back to expose its cavity and the shell-muscles, 
a, a, the divided portions of the mantle ; b, b, sheaths of the tentacles ; c, c, the 
funnel; d, its valve ; e, e, shell-muscles;/,/, their terminations or sur- 
faces of attacliment ; (j, the transverse fibres connecting them. 
A comparison of the muscular impressions of Cceloaaatilus with 
those of the recent Nautilus points to the conclusion that the 
animal must in the former have been fixed more firmly in its shell 
than in the latter, and furthermore the shell-muscles in Coelo- 
nautihis were in all probability not limited to the sides of the 
animal, as in the recent Nautilus^ but completely encircled it. 
