REPOET OX THE PTEROPODA. 
41 
The first two authors hare limited themselves to a description of the ganglia, without 
noticing the various nerves which proceed from them ; the last has given no figure. 
Cuvier recognised the four pairs of ganglia which make up the central nervous system of 
Pneumonoderma ; but nothing further is to be learned from his description and figure. 
Yan Beneden has given a better figure of the central nervous system, without, however, 
distinguishing the cerebro-pleural and eerebro-pedal connectives, which he regards as 
forming only a single trunk. Gegenbaur confines himself to a very brief description, in 
the course of which he remarks that the acetabuliferous appendages are innervated by 
the pedal ganglia ! Lastly, the description of Souleyet is much more exact, and my own 
researches on the nervous system of Pneumonoderma have shown that his figures are 
exceedingly accurate. 
On the other hand, the comparative examination which I have made of the genera 
Dexiobranchaea, Spongiobranchaea, and Pneumonoderma has demonstrated that the 
central nervous system is essentially similar in composition in these three genera ; so 
much so, indeed, that to describe one of them is to describe all. 
The cephalic portion of the Pneumonodermatidae being somewhat elongated (see PI. V. 
%• 1). as also the anterior part of the digestive tract, the central nervous system is 
situated relatively farther back than in the other Gymnosomata. As in all these it is 
composed of eight ganglia (PI. V. fig. 1) disposed in pairs. For information regarding 
the form and relative dimensions of these ganglia I may refer to the figures, from which 
it may be obtained more readily than from even a long description. 
Three of the pairs of connectives — cerebro-pleural (PI. V. fig. 1, e), cerebro-pedal (f), 
and pleuro-visceral (g) are here rather long, in such a manner that the four pairs of 
ganglia are less concentrated than in other families. It is in Dexiobranchaea, that the 
connectives (especially the cerebro-pleural and cerebro-pedal) are the longest, and that 
the concentration of the ganglia is least marked ; this agrees well with the other archaic 
characters of this genus. 
I. The cerebral ganglia (PI. V. fig. 1, a) are slightly elongated transversely, and almost 
in apposition. In Spongiobranchaea there is a small cerebral commissure, more appre- 
ciable than in Pneumonoderma. 
From each cerebral ganglion issue three connectives — cerebro-pleural (PI. IV. 
fig. 9, /), cerebro-pedal (g), and cerebro-buccal (i), the first being the strongest. The 
last is the most slender, and arises from the oesophageal face of the ganglion (see PI. IV. 
fig. 9, i, and PI. V. fig. 3 ,j), whilst the cerebro-pleural and cerebro-pedal connectives 
issue from the lateral surface, one behind the other, the latter being the anterior. 
The anterior part of each cerebral ganglion gives rise to three nerves, or more 
correctly to two, for the two lateral nerves (PI. V. fig. 1 , j and Jc) arise by a common 
trunk. 
1. The median nerve ( l ) passes to the proboscis, the buccal opening, and the lips. 
(ZOOL. CHALL. EXP. — PART LXVI. — 1888.) Uuu 6 
