MOLLUSCA — CEPHALOPODS 



253 



In this natural creeping position the soft body is nearly hori- 

 zontal, though curving upward posteriorly. The head is 

 anterior, the siphuncle is posterior, 

 the funnel is ventral and the coil 

 of the shell is dorsal. 



Shell. — The adult shell pos- 

 sesses about two and a half 

 whorls ; the exterior is marked 

 with alternating bands of reddish 

 brown and white. Internally, 

 the shell as previously stated is 

 made up of a series of chambers 

 separated by septa. Pelseneer 

 suggests that each septum shut- 

 ting off a chamber " is formed 

 during a period of quiescence, 

 probably after the reproductive 

 act, when the visceral mass of the 

 Nautilus may be slightly shrunk." 

 The thin glandular skin covering 

 the posterior portion of the animal 

 secretes the septum, probably first 

 as a membrane of conchiolin, 

 which later becomes impregnated with lime. The lines of union 

 of these septa with the outside walls of the shell are called 

 sutures. These are invisible externally except where the outer 

 surface has been worn off. (Compare with Fig. 116, su.) The 

 sutures are slightly undulating, possessing a small ventral lobe 

 (an undulation convex away from the aperture) and broad lateral 

 saddles (undulations convex toward the aperture). (Compare 

 with Fig. 117, /./., Is.) 



Thus as the animal grows it moves forward, building about it 

 an ever enlarging shell and at certain intervals shutting off by a 

 septum the empty shell, which it thus leaves behind, and main- 

 taining connection with the empty portion only by the siphon 



Fig. 113. — Nautilus macrompha- 

 lus, creeping upon a horizontal 

 surface. Note that the coil of the 

 shell is anterior (compare with 

 Fig. no), e., eye ; ho., hood ; o'., 

 0"., anterior and posterior ocular 

 tentacles ; sh., shell ; te., tenta- 

 cles. (Redrawn from Lankester's 

 Zoology after Willey.) 



