364 



LOWER INVERTEBRATES. 



The nervous system is greatly concentrated, the cerebral, pleural, and pedal gang, 

 lia being placed close together around the throat. True organs of smell have been 

 found only in the Nautilus, where they occupy the normal position at the bases of the 

 oills. In this genus, as well as in all others, close by the eyes are the openings to a 

 pair of j)its, which have been supposed to have olfactory functions. The ears are two 

 in number, and are small pits or vesicles which in the adult become entirely shut off 

 from the exterior. They are placed in the sides of the head below the eyes, and each 

 contains a single otolith. 



In the Nautilus the eye has not reached that high development found in the squids 

 and cuttle-fish, but in fact it is extremely simple. On either 

 side of the head are two prominences which contain the eyes. 

 In the centre of each is a minute hole communicating with a 

 large internal chamber filled with sea-water. The inner wall 

 of this is lined by the retina, in which the nerves terminate. 

 As will be seen, tliis eye is extremely simple. There is no 

 lens, no cornea; nothing but the features enumerated. All 

 are familiar with the fact that a lens is not necessary for the 

 production of an image. If we admit the light to a darkened 

 room, by means of a very small hole, so as to prevent any superimposition of images, a 

 picture of external objects can be thrown on a screen. This is the physiology of the 

 eye of the Nautilus. In the Dibranchiata the visual organ is vei-y complex, and it is 

 to be noticed that in its development it passes through a stage closely similar to 

 that which persists through life in the Nautilus. With development it goes farther. 



Fig. 470. — Du-igram of the eye 

 of Nautilus; c, cavity; i, 

 integument; 7i, nerves; r, 

 retina. 



Fig. 477. —Section tUrougb the head of a young Loligo to show the structure of tlie eyes; c. ceso- 

 pliagus; r/, e, pedal ganglion; y, optic ganglion; A, ganglionic layer of retina; i, layer of rods; 

 I:, posterior chamber of the eye; /, lens; m, anterior chamber; o, cornea; p, iris. 



and the result is an optical organ which presents some startling analogies to that 

 found in the vertebrates. When carefully studied it is seen that this resemblance is 

 superficial, and that fundamentally the two are entirely different. In both the eye 

 is provided with a cornea, then comes an iris with a circular pupil, and next a lens 

 dividing the cavity into two chambers. Here, however, the correspondence stops. 

 In the squid the posterior lining of the inner chamber is the retina, with its rods and 

 cones, and then comes the ganglionic layer. In the vertebrates the relations of these 

 two are reversed. Embryology reinforces these differences, and shows that the eye of 

 the cephalopod is greatly different in its development from that of the vertebrate. 



