REPORT ON SPIRULA. 
17 
posteriorly, thin threads proceeding towards the heart and the kidneys, and transversely 
to the right and left a branchial nerve which penetrates into the branchial axis, without 
swelling, at its base, into an “ olivary ” ganglion . 1 
As to the pallial nerves, they proceed from lateral portions (pleural ganglia) of the 
posterior infra-oesophageal mass; they are exceedingly 
strong (PI. Y. pl.n. ; Fig. N, x), directed towards the 
dorsal side, traversing the wall of the “mesosoma” and 
arriving at the mantle, at the very point where the latter 
is joined to this mesosoma; at this point they form, as in 
all the Dibranchiates, “stellate” pallial ganglia, without 
being bifurcated before entering into these . 2 
The two stellate ganglia are united by a curved commis¬ 
sure (with anterior concavity), very thin (Fig. N, iv), but 
appearing stronger because it is joined to a venous trunk, 
which the nervous cord accompanies through all its course, 
passing to the surface of the mesosoma covering the anterior 
extremity of the shell; from the middle of the curve there 
arise a vein and a nerve directed forwards (Fig. N, ii), 
which, passing over the dorsal margin of the shell opening, 
become recurrent and run along, following the median line, 
the part of the mantle contained in the last chamber of the 
shell, the venous trunk emptying itself into the vena cava 
(see Circulatory System). 
This commissure, with its median nerve, ought to be 
considered as formed by the two original pallial nerves of 
Cephalopods fused together, and the larger pallial nerves 
situated beyond the stellate ganglia (innervating especially the fins; Fig. N, iii) are 
adventitious formations necessitated by the great development of the margins of the 
1 Homologue of the osphradial ganglion of other Molluscs, but not functionally equivalent; in various 
Cephalopods this “ olivary ” or branchial ganglion is rather superficial (example, Eledone ), but it does not send 
nervous fibres to the subjacent epithelium, and this does not contain any special sensorial cells. The fact 
that the osphradium has not yet taken its rise in Cephalopods is explained by the presence of the olfactory 
rhinophoric fossa at the entrance of the pallial cavity on the side where respiratory water enters, which 
renders a second olfactory organ (osphradium properly so-called) useless. The interbranchial papilla of 
Nautilus, identified by Lankester and Bourne {Quart. Journ. Micr. Sei., vol. xxiii. p. 343, 1883), with the 
osphradium of Molluscs, is not itself a sensorial organ, and does not contain any ganglion homologue of 
the osphradial centre. But it appears to constitute simply a protective apparatus of a sensorial region, 
situated on the portion of the internal face of the mantle immediately anterior to the papilla, which is 
inclined above it; in this region a branch of the branchial nerve is divided into very numerous twigs, which 
send a great many bundles to the epithelium. 
2 As well as in “ Loligopsis ” ( Leacliia ), according to Grant (On the structure and characters of 
Loligopsis, Trans. Zool. Soc. London, vol. i., 1833). 
(ZOOL. CHALL. EXP.—PART LXXXIII.— 1894.) 
Fig. N .—Spirula reticulata, dorsal view, 
the anterior part of the mantle re¬ 
moved on this side (the internal part 
of the shell shown by a broken line); 
x 4. i, mantle edge; ii, median 
nerve ; iii, nerve of the fin ; iv, oom- 
missure of the stellate ganglia; v, fin ; 
vi, terminal disk ; vii, dorsal external 
part of the shell; viii, internal part 
of the shell; ix, stellate ganglion ; x, 
pallial nerve. 
3 
