The Cambridge natural history . fter Hoyle, x \). Order Tetrabranchiata Cephalopoda with four branchiae and four kidneys; animalinhabitino- the last cliiiiiilici of ;iu external multiloculai- shell; 192 CEPHALOPODA TETRABRANCHIATA funnel consisting of two separate lobes; tentacles ninnerons,without suckers or hooks; no ink-sac. The shell consists of two layers, the outer being porcellanous,and the inner, as well as the walls of the chamlters or septanacreous. The septa vary greatly in shape. In most of thea^autiloidea they are regularly curved, as in Nautilus, orstraight, as in Orthoceras, but
The Cambridge natural history . fter Hoyle, x \). Order Tetrabranchiata Cephalopoda with four branchiae and four kidneys; animalinhabitino- the last cliiiiiilici of ;iu external multiloculai- shell; 192 CEPHALOPODA TETRABRANCHIATA funnel consisting of two separate lobes; tentacles ninnerons,without suckers or hooks; no ink-sac. The shell consists of two layers, the outer being porcellanous,and the inner, as well as the walls of the chamlters or septanacreous. The septa vary greatly in shape. In most of thea^autiloidea they are regularly curved, as in Nautilus, orstraight, as in Orthoceras, but in the Animonoidea they are oftenexceedingly complex. The edge of the septum, where it uniteswith the shell-wall, is called the suture, and the sutural line,which is not seen until the porcellanous layer is removed, variesin shape with the septum. The septa are traversed by a membranous tube known as thesiphuncle, which in Nautilus is said by Owen to connect ulti-mately with the pericardium. The sejital necks, or short tubular. Fig. 252.—Xautilus pomjjiUus L., in section, sliowing the septa (s, s), tlie septal necks{, ), the siphuncle dotted in (si), and the large body chamber {ch). prolongations of the septa where they are perforated by thesiphuncle, are in the great majority of the Nautiloidea directedbackwards (Fig. 252), , they project from the front wall ofeach chamber, while in nearly all Animonoidea they are directedforwards. When the sipliuncle is narrow, as in the Ammon-oidea, it is simple, Init when wide, as in many of the Nautiloidea, NAUTILOIDEA 393 its walls are often thickened by the deposition of masses of cal-careous matter, or by rings and radiating lamellae of the samematerial. In position, the siphuncle is sometimes central, some-times sub-central, sometimes (Ammonoidea)marginal. In some cases its position is /y^^^^^^^believed to change during the growth of /(11 n^<;^i^^^^^the individual. The precise object served fv^M|\^ ^rl ^^^by the siphunc
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
