XII 



PHYLUM MOLLUSCA 



765 



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 

 microscopic 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. 685, o.t.) 

 are supposed to act as accessory olfactory organs. The osphradia 

 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 excep- 

 tion perhaps of Patella (p. 726). 

 Each is of the shape of a 

 saucer, attached to the head 

 f 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 



post aort \ t hi s . to the central aperture. 



In the interior of the cup is 

 neither lens, vitreous humour, 

 FIG. 693.- Nautilus pompiiius, male repro- nor iris. The sea-water, pas- 



ductive organs, ace. vesicula seminalis ; eff. ,, , ,1 i 1 



vess. efferent branchial vessels ; /. gen. op. Sing in tnrOUgn tne Central 



left genital opening; post. aort. posterior aT AT fnrp rlirppfhr Kafhp fhp 



aorta ; pyr. pyriform appendage ; r. gen. ^pei 1C, (. ^biy UdUll 



op. right genital [opening ; sp. s spermato- retina, which is spread over 



phore-sac ; test, testis ; vent, ventricle. 



the interior in a thick layer. 



Reproductive Organs. The gonad (testis, Fig. 693, test, or 

 ovary, Fig. 694, 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 seminalis, 

 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 

 a receptacle, the spermatophoral sac or Needham's sac (sp. s.), and 

 opens, nearly in the middle line at the end of a prominence the 



VOL. I 3 C 



