20 Guide to the Invertehrata. 



Nautilus, in diameter were seen by Dr. Bigsby standing out in bold relief 

 GALLEEY from the cliffs of Silurian rock on Drumraond Island, Lake Huron, 

 ^^^* North America, only faint traces of the chambers, in one or two 

 7 2 8*'*^®^ instances, being visible. 



In Orthoceras there is evidence of a continued connection through 

 these large and complex siphonal tubes with the whole series of 

 body-chambers ; but if this were the case in the Silurian Cephalo- 

 poda, it is not so in Nautilus^ there being no connection whatever 

 between the siphuncle and the disused and deserted chambers of 

 the shell, which are hermetically sealed up. They are the left- 

 off portions of the cephalopod's habitation, just as much as the 

 lower part of the skeleton of a coral is shut off from the upper 

 part inhabited by the living zoophyte. In EuomphaluSy a Silurian 

 gasteropod, portions of the whorls of the shell, not needed by the 

 animal, are similarly closed by a shelly septum or division. 



The modern Nautilus is rather a sluggish animal, living mostly 

 at the bottom in deep water, where it may perhaps crawl by means 



Fig. 40. — Section of Nautilus striatus, Sby., to show the septa and the 

 siphuncle. Lias : Charmouth, Dorset. (Much reduced.) 



of its tentacles, mouth downwards, feeding upon small crabs 

 and Echinoids. The fishermen of Eiji, the New Hebrides, etc., 

 catch it in their crab-pots, which it enters. Wiley says that it 

 habitually swims backwards, after the manner of other Cephalopods, 

 by discharging the water from its funnel. 



This Tetrabranch, or four-gilled division, represented to-day by 

 a single living form, the " pearly Nautilus," was formerly most 

 abundant almost all over the globe, shells referred to it being 

 found in rocks of all ages from the Silurian to the newer Tertiary 

 deposits. The extinct genera were extremely varied in form, but 

 nearly all possessed shells of the same compact nature as their 



