MOLLUSCS. 113 



been advanced. It would appear, however, that there is a periodicity in 

 growth, and at intervals the animal moves forward, building a new partition 

 behind it. The chambers are filled with a gas which differs from common 

 air in having a greater percentage of nitrogen ; and the suggestion at 

 once arises that these chambers thus have a buoyant function; but why 

 the nautilus needs buoying up more than any other mollusc does not at 

 once appear. The eye, too, is interesting. Every boy is familiar with the 

 'pin-hole camera.' He takes a pill-box, punctures the bottom with a 

 needle, replaces the cover with a bit of ground glass or tissue paper, and 

 then sees on the surface external objects in a reversed position. The eye 

 of the nautilus is built on exactly the same 

 plan. As shown in the figure, there is the 

 same small aperture, while the ground glass is 

 replaced by the retina, which is in connection 

 with the terminations of the optic nerve. The 

 cavity of the eye is in full communication with 

 the exterior, and, of course, is filled with salt 

 water. It is interesting to compare this figure 



01 ° Fig. 107. —Section of the eye of the 



With that of the eve of the quahoo- (Fk»\ 78) On nautilus, formed on the principle of 



^ x o v o / the pin-hole camera. At r is the 



a preceding page, and see realh' how slight are retina; n, the nerves The opening 



o 1 o ' j o above c is minute, and no lens exists. 



the modifications of the simpler plan. 



Of the forms with but two gills there are two types : in the one, the 

 cuttle-fishes, there are eight arms surrounding the mouth ; in the other, 

 the squids, there are ten of these arms, and associated with this difference 

 in number are considerable differences in form of bod}*, as will be seen 

 by our cuts. In connection with the reproduction of these forms a 

 very peculiar fact must be noted. Years ago the great French naturalist, 

 Cuvier, found adhering to the body of a cuttle-fish which fell into his hands, 

 a peculiar worm-like body which he had no doubt was a parasitic woim, 

 and which he at once called Hectocotyie in allusion to the eight suckers by 

 which it adhered to its host. Now the truth is known, and it has been 

 placed beyond a~ doubt that this seeming parasite in reality is not a parasite 

 at all, but is merely one of the arms of a male squid or cuttle-fish peculiarly 

 modified for reproductive purposes. At certain times of the "year the male 

 matures its supply of milt, and then a packet of it is forced into one of 

 the arms, which is then ready to be cast off and to adhere to the female, 

 thus effecting a fertilization of the eggs. Before the next breeding season 

 a new arm grows to take the place of the lost one. 



"VVe have just alluded to the suckers of the hectocotyie, and since these 

 are very characteristic of the squids and cuttle-fishes, we may be pardoned 

 a word concerning them. On all of the arms of scmids and cuttles occur 



