292 THE CEPHALOPODA 
Other differentiated muscular bundles may be recognised ; they are 
mostly due to the specialisation of the funnel. 
In the Tetrabranchia (Nautilus) the mantle is covered by an 
external shell, which is partly overlapped by a small dorsal pallial 
lobe (Fig. 270, d): the retractor muscles of the head and foot are | 
inserted symmetrically on either side on the internal surface of 
this shell. The female Argonauta also bears an external shell which 
covers the mantle, but has no muscular attachments and is not 
homologous with the shells of other Cephalopods: it does not 
originate from a pre-conchylian invagination or shell-gland, but is 
of pedal origin, and is only formed some ten or twelve days after 
birth by the palmate extremities of the two dorsal arms. The 
animal is not attached to this shell. 
In all other Cephalopoda the shell is covered over by the 
mantle, or at least is partly covered in Spirula (Fig. 295). The 
shell therefore is internal, and often is rudimentary, as in the 
majority of Decapoda, or it may be nearly obsolete, as in the 
Octopoda. The shell of living and fossil Nautiloidea, of Ammonoidea, 
Spirula (Fig. 268, sp), and of various fossil Dibranchia, such as the 
Belemnitidae, Spirulirostra (Fig. 262, C), ete.,is provided with internal 
septa, disposed perpendicularly to the axis of the coil. It is only 
the last of the chambers thus formed that is occupied by the body 
of the animal, but a prolongation of the pallial integument known as 
the pallial siphuncle (Fig. 270, /) extends back to the initial chamber 
of the shell, and is enclosed in a calcareous tube or shell siphuncle 
which perforates all the septa (Fig. 268, sz). This pallial siphuncle 
does not communicate with the coelomic cavity: in Nautilus and 
Spirula it is a simple vascular vermiform process of the mantle, 
whose cavity consists of a venous sinus and whose wall contains a 
ramification of the pallial artery. It apparently plays a part in 
the hydrostatic function. At the point where the shell siphuncle 
traverses each septum it is generally surrounded by a small 
reduplication of the latter, forming the so-called siphuncular neck. 
The chambers traversed by the siphuncle do not communicate with 
one another nor with the shell siphuncle: they are filled with a 
nitrogenous gas and form a hydrostatic apparatus. 
The external multilocular shell is straight in some palaeozoic 
Nautiloidea (Orthoceras), but in the majority of Tetrabranchia it is 
arcuate or more or less completely coiled in such a manner as to 
form a discoidal shell whose whorls are all in the same plane. In 
the majority of Tetrabranchia (Nautilus, Fig. 270) the coil is exo- 
gastric, that is to say, it is turned towards the dorsal aspect, but 
in some forms, ¢.g. Phragmoceras, Cyrtoceras, Ptenoceras (Fig. 261, B), 
it is turned towards the ventral side and is therefore endogastric ; 
the direction of the coil cannot be determined by the position of 
the siphuncle, which traverses the septa at various points, but by 
