CEPHALOPODA.] MOLLUSCA G79 of the two nephridia to form one sac is still more obvious, since the ventral portions are united. In Octopus the nephridia are quite separate. Tegumental pores have not been described in Nautilus, but exist in Dibranchiata, and have been (probably erroneously, but further investigation is needed) supposed to introduce water into the vascular system. A pair of Fig. 114. Fios. 113, 114. Nerve-centres of Octopus. Figure 113 gives a view from the dorsal aspect, figure 114 one from the ventral aspect, buc, the buccal mass ; ped, pedal ganglion ; opt, optic ganglion ; cer, cerebral ganglion ; pi, pleural ganglion ; vise, visceral ganglion ; cus, oesophagus ; /, foramen in the nerve- mass formed by pedal, pleural, and visceral ganglion-pairs, traversed by a blood-vessel. such pores leading into sub-tegumental spaces of consider able area, the nature of which is imperfectly known, exist on the back of the head in Philonexis, Tremoctopus, and Argonauta. At the base of the arms and mouth four such pores are found in Histioteuthis and Ommastrephes, six in Sepia, Loligo, Onychoteuthis. Lastly, a pair of such pores are found in the Decapoda at the base of the long arms, lead ing into an extensive sub-tegumental pouch on each side of the head into which the long arms can be, and usually are, withdrawn. In Sepia, Sepiola, and Ros- sia the whole arm is coiled up in these sacs ; in Loligo only a part of it is so ; in Histio teuthis, Ommastrephes, and Onychoteuthis, the sacs are quite small and do not admit the arms. Nervous System. Nautilus, like the other Cephalopoda (e.g., Pneu- modermon, fig. 87 ; Octopus, fig. 113), ex hibits a great concentra tion of the typical Mol- luscan ganglia, as shown in fig. 112. The gan glia take on a band-like form, and are but little differentiated from their commissures and con nectives, an archaic condition reminding us of Chiton. The special optic out growth of the cerebral ganglion, the optic ganglion (fig. 112, o), is characteristic of the big-eyed Siphonopoda. The cerebral ganglion-pair (a) lying above the oesophagus Fm. 115. Lateral view of the nervous centres and nerves of the right side of Octopus vul- jraris (from a drawing by A. G. Bourne), bg, buccal ganglion ; cer., cerebral ganglion ; ped., pedal ganglion ; pi., pleural, and vise., visceral region of the pleuro-visceral ganglion; gang, stell., the right stellate ganglion of the mantle connected by a nerve to the pleural portion ; n.visc., the right visceral nerve ; n.olf., its (probably) olfactory branches ; n. br., its branchial branches. is connected with two sub-oesophageal ganglion -pairs of band-like form. The anterior of these is the pedal b, b, and supplies the fore-foot with nerves t , t, as also the mid-foot (siphon). The hinder band is the visceral and pleural pair fused (compare fig. 112 with fig. 87, and especially with the typical Euthyneurous nervous system of Limnams, fig. 22) ; from its pleural portion nerves pass to the mantle, from its visceral portion nerves to the branchiae and genital ganglion (d in fig. 112), and in immediate connexion with the latter is a nerve to the osphradium or olfactory papilla. No buccal ganglia have been observed in Nautilus, nor has an enteric nervous system been described in this animal, though both attain a special development in the Dibranchiata. The figures (114 and 115) representing the nerve-centres of Octopus serve to exhibit the disposition of these parts in the Dibranchiata. The ganglia are more distinctly swollen than in Nautilus. In Octopus an infra-buccal ganglion-pair are present cor responding to the buccal ganglion-pair of Gastropoda. In Decapoda a supra-buccal ganglion -pair connected with these are also developed. Instead of the numerous radi ating pallial nerves of Nautilus, we have in the Dibran chiata on each side (right and left) a large pleural erve passing from the pleural portion of the pleuro- visceral ganglion to the mantle, where it enlarges to form the stellate ganglion. From each stellate ganglion nerves radiate to supply the powerful muscles of the mantle-skirt. The nerves from the visceral portion of the pleuro-visceral ganglion have the same course as in Nautilus, but no osphradial papilla is present. An enteric nervous system is richly developed in the Dibranchiata, connected with the somatic nervous centres through the buccal ganglia, as in the Arthropoda through the stomato-gastric ganglia, and anastomosing with deep branches of the vis ceral nerves of the viscero-pleural ganglion-pair. It has been especially described by Hancock (39) in Omma strephes. Upon the stomach it forms a single large and readily-detected gastric ganglion. It is questionable as to how far this and the " caval ganglion " formed in some Decapoda by branches of the visceral nerves which accom pany the vena cava are to be considered as the equivalents of the "abdominal ganglion," which in a typical Gastropod nervous system lies in the middle of the visceral nerve-loop or commissure, having the right and left visceral ganglia on either side of it, separated by a greater or less length of visceral nerve-cord (see figs. 20, 21, 22). There can be little doubt that the enteric nervous system is much more developed in the Dibranchiata than in other Mollusca, and that it effects a fusion with the typical " visceral " cords more extensive than obtains even in Gastropoda, where such a fusion no doubt must also be admitted. Special Sense-Organs. Nautilus possesses a pair of osphradial papillae (fig. 101, olf) corresponding in position and innervation to Spengel s organ placed at the base of the ctenidia (branchiae) in all classes of Mollusca. This organ has not been detected in other Siphonopoda. In Ptero- poda it is well developed as a single ciliated pit, although the ctenidia are in that group aborted (fig. 87, Osp.}. Nautilus possesses other olfactory organs in the region of the head. Just below the eye is a small triangular process (not seen in our figures), having the structure of a shortened and highly-modified tentacle and sheath. By Valenciennes, who is followed by Kef erstein, this is regarded as an olfactory organ. The large nerve which runs to this organ originates from the point of juncture of the pedal with the optic ganglion. The lamelliform organ upon the inner inferior tentacular lobe of Nautilus is possibly also olfactory in function. In Dibranchs behind the eye is a pit or open canal supplied by a nerve corresponding in
origin to the olfactory nerve of Nautilus above mentioned.