XII PHYLUM MOLLUSCA 787 



Nervous System.— Nautilus difFers strikingly from Sepia, and 

 somewhat resembles Chiton (p. 716, Fig. 604) in the form assumed 

 by the central parts of the nervous system (Fig. 681, cer. g.), 

 distinct ganglia being absent. A very thick nerve-collar^ 

 the posterior portion of which is double, surrounds the oesophagus 

 just behind the buccal mass. The anterior part of the collar 

 (Fig. 681, cer. //.) represents the cerebral ganglia, the oral 

 portion of the posterior part the pedal {ped. g.), the aboral 

 portion the pleuro- visceral {pi. g.) ; while the lateral parts, not 

 distinctly marked off from the rest, represent the cerebro-pedal and 

 cerebro-pleural connectives. From the cerebral ganglia pass 

 nerves to the buccal mass, to the olfactory organs {olf. n.) and 

 the statocysts, and a pair of very thick optic nerves supply the 

 eyes (opt. n.). The pedal ganglion gives off numerous nerves 

 to the tentacles and the funnel, and from the pleuro-visceral arise 

 pallial and visceral nerves. 



Sense Organs — The statocysts (otocysts) are a pair of sacs 

 embedded in recesses close to the cerebral ganglia, but not enclosed 

 in the cartilage of the endoskeleton ; each contains a number of 

 microscropic statocones. An olfactory function is ascribed to a. 

 process (the rhinophore) with a ciliated pit at its base, situated on 

 the aboral side of the eye. The ophthalmic tentacles (Fig. 676, o.t.) 

 are supposed to act as accessory olfactory organs. The osphradia 

 (p. 781) contain ganglion-cells, are beset with sensory cilia, and 

 are undoubtedly organs of special sense. 



The eyes, situated at the sides of the head, are very large but 

 extremely simple in structure, presenting a marked contrast to 

 those of Sepia, and scarcely comparable to those of any other 

 animal with the exception perhaps of Patella (p. 746). Each is 

 of the shape of a saucer, attached to the head by its convex side by 

 means of a short thick stalk, the mouth being closed in by a slightly 

 convex disc, with a circular aperture at about its .centre. A 

 slightly raised rim runs round close to the margin on the posterior 

 half, and a narrow groove extends inwards from this to the central 

 aperture. In the interior of the cup is neither lens, vitreous 

 humour, nor iris. The sea-water, passing in through the central 

 aperture, directly bathes the retina, which is spread over the 

 interior in a thick layer. 



Reproductive Organs The gonad (testis. Fig. 684, test., or 



ovary, Fig. 685, ov.), like that of Sepia, is single and median, enclosed 

 in a special sac towards the aboral end of the body. The duct 

 is paired in both sexes, but in both the right alone appears to 

 be functional. In the male a large glandular vesicula seniinalis, 

 in which the spermatophores are formed {ace.) is connected with 

 the right duct, and this appears to be represented on the left-hand 

 side by a vestige — the so-called pyriform sac {pyr.), situated close 

 to the ventricle. The distal part-of the right duct dilates to form 



