44 
THE VOYAGE OF H.M.S. CHALLENGER. 
From the anterior extremity of the pleural ganglion issues the cerebro-pleural con- 
nective, and from its posterior extremity the pleuro- visceral connective (somewhat 
elongated in Pneumonoderma), which leads to the corresponding ganglion of the fourth 
pair. 
IV. This fourth pair is composed of two ganglia (PI. IV. fig. 9, d) in close 
apposition and almost spherical. Hitherto we have seen in the nervous system of the 
Pneumonodermatidse an absolute symmetry in the size of the two ganglia of the same 
pair (cerebral, pleural, and pedal ganglia), and in the number of nerves which they give 
off. Now, however, this symmetry ceases. 
An attentive examination shows that these two ganglia are slightly unequal, the 
left being the larger. But the asymmetry becomes more striking when we consider the 
nerves given off from them. The right ganglion only gives origin to one nerve, while 
three spring from the left hand one, a lateral nerve and two posterior almost median 
nerves. This asymmetry is found in all the Gymnosomata, and has only been noticed 
by Souleyet, who, however, did not attach any importance to it. Spengel 1 is wrong in 
attempting to modify Souleyet’s figure so as to render the visceral nerves symmetrical in 
their origin. 
1. The nerve from the right visceral ganglion (1), which is symmetrical with 
the lateral nerve of the left ganglion, supplies the right half of the visceral sac 
and the osphradium. This is constituted as in Clionopsis (PI. IV. fig. 10, a) by 
a ganglionic pad occupying the antero-lateral aDgle of the cloacal depression ; a 
divided branch of the nerve (1) passes to it. 
2 and 3. The two posterior nerves of the right ganglion ( 2 and 3) innervate the 
viscera (genital organs, heart, kidney) and the gills. 
4. Finally, the lateral nerve of the left ganglion (4) arises near the connective, and 
supplies the left half of the visceral sac. 
V. The buccal ganglia, seen for the first time by van Beneden, are situated below the 
oesophagus, between the two hook-sacs, a little behind the point where the salivary 
glands open. They are in close juxtaposition, and each of them gives origin anteriorly 
to a filament on which is situated an accessory ganglion ; these threads innervate the 
salivary glands. 
The other nerves which arise from the buccal ganglia are five in number, as is shown 
in the figures of Souleyet and van Beneden — one azygous anterior nerve arising from 
the point where the two buccal ganglia are in contact, and innervating the radula ; and 
also a lateral and a posterior nerve from each ganglion. The former supplies the walls 
of the buccal mass, and the latter the hook-sacs. 
1 Die Geruchsorgane und das Nervensystem der Mollusken, Zeitschr.f. wiss. Zool., Bd. xxxr. pi. xvii. fig. 10. 
