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), etc., 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, 1) 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, si). 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, e.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 



