56 
THE VOYAGE OF H.M.S. CHALLENGER. 
The central nervous system is formed of eight ganglia, except in Halopsyche, where 
there are only seven. The cerebral ganglia are closely approximated to each other, and 
are the only supracesophageal ones ; I lay stress on this point, because so recently as 
1877 Garner has stated 1 that Clione possesses six ganglia, of which, in contradistinction 
to the arrangement in Pneumonoderma , four are above the oesophagus. The pleural 
ganglia, distinct from the cerebral, are close to the pedal. 
III. SUMMARY ON THE PTEROPODA. 
In the Pteropoda the lateral portions of the foot are all modified into fins. The jaws 
are lateral and paired. There are salivary glands. The stomach has “ horny ” plates in 
the adult condition, or only during the larval stages (Gymnosomata). The radula has in 
the same transverse row lateral teeth, which resemble each other in form, differing only 
in size. 
The flexure of the intestine is not neural, but resembles that of the Gastropods, which 
is improperly called dorsal, and would be more correctly termed lateral. 
The heart is lateral and the excretory organ azygous. The pericardium is isolated from 
the circulation. This is a statement of importance, because even in 1882 Claus 2 stated 
that water enters the circulatory system by the kidney and pericardium ; and because 
this same author states that in the Pteropoda the blood goes from the respiratory organs 
to the heart by way of the pericardial “sinus,” even though so long since as 1857 
Herman Muller showed the absence of corpuscles in the fluid of the pericardium. 
The hermaphrodite genital gland has a single efferent duct and a single common 
genital aperture, from which a seminal groove leads to the copulatory organ, situated in 
the cephalic region. 
The nervous system is characterised by the asymmetry of the visceral portion. The 
pedal ganglia have a double commissure. 
The Pteropoda are thus essentially characterised by the asymmetry of their internal 
organisation, combined with the symmetry of their external form. 
1 Malacological Notes, Ann. and Mag. Nat. Hist., ser. 4, vol. xix. pp. 372, 373. 
2 Grundziige der Zoologie, t. ii. p. 37, 1882. 
