142 



MOLLUSCA 



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 



CtK 



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, hue, the buccal mass ; 

 fed, pedal ganglion ; opt, optic ganglion ; cer, cerebral ganglion ; pi, pleura! 

 ganglion ; vise, visceral ganglion ; ois, 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are, withdrawn.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- _ 



..* " , Fro. 115. Lateral view of the nervous centres 



lUSCan ganglia, as Shown and nerves of the right side of Octopus vul- 



in fit* 112 The gan- 9 (from a drawing by A. G Bourne). %, 



n . iiw. buccal ganglion; cer., cerebral ganglion; 



glia take On a band-like ptd., pedal ganglion ; pi, pleural, and vise., 



c j r, 4. i-4.ii visceral region of the pleuro-visceral ganglion; 

 lOrm, and are but little 



gang . ste i(^ 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 brauchial tranches. 



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 



is connected with two sub-cesophageal 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 

 [>leural pair fused (compare fig. 112 with fig. 87, and 

 especially with the typical Euthyneurous nervous system 

 of Limnseus, 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 papilla (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 Keferstein, 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. 



