62 
THE VOYAGE OF H.M.S. CHALLENGER. 
the kidney of the Pteropoda is azygous and asymmetrical, whilst in the Cephalopoda 
there are two (four in Nautilus) symmetrical kidneys, isolated or in communication. 
C. As regards the genital organs, we may first remark that the Cephalopods are 
dioecious, whilst the Pteropods are hermaphrodite. 
The genital ducts differ very widely in the two groups — in the Cephalopoda the genital 
gland is isolated in a kind of coelomic space, and has no direct continuity with the genital 
duct, which is only continuous with the wall of the cavity just mentioned. In the 
Pteropoda, on the contrary, the genital duct is continuous with the envelope of the gland, 
and, further, these animals have only a single asymmetrical genital duct, whilst in the 
Cephalopoda there are numerous indications of paired symmetrical ducts — in Nautilus in 
both sexes , 1 in the females of the Octopoda and of Ommatostrephes ; and finally in 
Rossia and Spirula, the oviduct is on the right (as is the functional oviduct in Nautilus ), 
whilst in the other Decapoda it is on the left, which proves that originally it was bilaterally 
symmetrical. 
Lastly, the copulatory organ of the Pteropoda is situated on the head, far from the 
genital aperture, and is not morphologically comparable with the penis of the Cephalopoda. 
D. Nervous System. — The nervous system of the Pteropoda differs from that of the 
Cephalopoda mainly in the absence of symmetry in the visceral portion. When there is 
an apparent symmetry in the disposition of the ganglionic visceral nervous elements, there 
is real asymmetry in the origin of the nerves. Furthermore, there is in the Cephalopoda 
a concentration of the central nervous system, so great that the commissures and con- 
nectives have almost disappeared ; and there exists in all these animals a pair of 
“ brachial ” ganglia which are entirely wanting in the Pteropoda, As regards the 
nervous system, as in other groups there is no indication of direct relation between the 
Pteropods and Cephalopods. 
The osphradium (Spengel’s olfactory organ) is paired in those Cephalopoda which 
possess it ; 2 it is unpaired in the Pteropoda. 
The otocysts of the Cephalopoda (except Nautilus ) enclose a single otolith ; in all 
the Pteropoda there are many otoliths. 
E. The Cephalopoda have two symmetrical columellar muscles (formed by the union 
of the retractor capitis and retractor pedis of either side), whilst the Pteropoda have only 
a single median columellar muscle. 
F. Ontogeny. — If after examining the adult animal we consider its ontogenetic 
development comparatively in the two groups, we find constant and clear differences. 
The segmentation of the ovum, which is complete in the Pteropoda, is only partial in the 
Cephalopoda. In the original development of the Pteropoda there is observed at the 
commencement the primitive symmetry of all Mollusca, but during the whole larval 
1 Ray Lankester and Bourne, On the Existence of Spengel’s Olfactory Organ and of Paired Genital Ducts in the 
Pearly Nautilus, Quart. Journ. Micr. Sci., vol. xxiii., N. S., p. 345. 
2 J6id.,p. 340; see also Zernoff, Ueber das Geruchsorgan der Cephalopoden, Bull. Soc. Nat.Moscou, 1869, p. 71,pls.i.,ii. 
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