REPORT OX THE PTEROPODA. 
87 
dorsal hooks of Notarchus (which have become specialised into the hook- 
sacs of the Gymnosomata) would represent the modified remains of the 
primitive horny ring. 
(iii.) Salivary Glands . — These organs in the Gymnosomata closely resemble the 
corresponding organs of the Aplysioid Tectibranchia ; they are narrow 
and elongated, and extend, gradually diminishing in diameter, from the 
distal extremity to their termination in the buccal mass, without any 
separation into a proper glandular portion and a distinct duct. 
(iv.) Stomach . — In the adult Gymnosomata this is unarmed. This absence of 
masticatory plates in the stomach is probably due to the exclusively car- 
nivorous diet of the Gymnosomata. Indeed the most carnivorous of the 
Bulloidea (e.g., Dondium ) are also without gastric plates. 
e. The Respiratory Organs. — The most primitive of the Gymnosomata (Pneumono- 
dermatidae) have a lateral gill (on the right side), the position and relations of which 
leave no doubt as to its homology with the gill of the Aplysioidea and of all the 
Tectibranchs (the posterior gill of Pneumonoderma, Spongiohranchsea, Clionopsis, and 
Notobranchsea being a new formation). This lateral gill, although simpler than that 
of the Aplysioidea, is analogous to it in its structure, for in Pneumonoderma it is also 
formed by the folding of a single lamella. 
f The Generative Organs . — The hermaphrodite genital gland of the Gymnosomata 
is arranged like that of all the Tectibranchs. The conformation of the genital duct in 
the Aplysioidea is exactly identical with that of the duct in the Gymnosomata, the 
accessory genital glands (albuminiparous and muciparous glands) and the receptaculum 
seminis being situated towards its extremity. The structure and position of the copu- 
latory organ also are the same both in the Aplysioidea and the Gymnosomata. 
g. The Nervous System . — There is almost absolute identity between the central 
nervous system of a Gymnosome {e.g., Spongiohranchsea, PI. V. fig. 3) and that of certain 
Aplysioidea, such as Notarchus 1 or Dolabellci (PI. V. fig. 2), and the central nervous 
system of other Aplysioidea only differs from that of the Gymnosomata in the elonga- 
tion of the pleuro-visceral connectives and the displacement backwards of the visceral 
ganglia. 
In the Gymnosomata and in all the Aplysioidea the cerebral ganglia are closely 
approximated on the dorsal aspect of the oesophagus ; the pleural ganglia are close to the 
pedal ganglia, so that the cerebro-pleural connectives are almost as long as the cerebro- 
pedal, and the pleuro-pedal connectives scarcely exist. A long and slender sub- 
oesophageal cerebral commissure (subcerebral commissure of von Jhering) also exists in 
both groups. 
1 Vayssiere, Recherches zoologiques et anatomiques sur les Mollusques Opistobranches clu Golfe de Marseille, 
loc. cit., pi. iv. figs. 94, 95. 
