370 PROFESSOR E. RAY LANKESTER, 
scribed and figured by Grube in Limnetis (‘ Archiv fir 
Naturgeschichte,’ 1853). 
In some very important respects the central nervous 
system of Apus appears to be more archaic and more nearly 
like that of the Chzetopod annelids than is that of any other 
Crustaceans. 
Recent researches on Planarians, Nemertines, Chetopoda, 
and Mollusca, point to the conclusion that the central 
nervous organs in all bilaterally symmetrical Coelomata are 
primarily (or after a certain stage of evolution) a pair of 
lateral cords, consisting of nerve ganglion cells and nerve 
fibres, which are at first united to one another across the 
middle line only by commissural fibres. The more abun- 
dant nerve cells in the prostomial portions of these primitive 
nerve-cords form a pair of archi-cerebral ganglia, which 
unite with one another to constitute the primitive przesto- 
mial ganglion or archi-cerebrum. Meanwhile other ganglia 
or concentrations of nerve cells are formed upon the 
portions of the lateral cords lying behind the mouth, and 
these apparently correspond in number to the segments (if 
segments exist), which are differentiated in the metastomial 
axis. These metastomial ganglia may become highly spe- 
cialised by the localising in them of all nerve cells, whilst 
the intermediate parts of the cords remain as simple bands 
of nerve fibres (Leeches, some Chzetopods, most Arthropods, 
most Mollusca), or nerve cells may still occur in quantity 
between the ganglia (Peripatus, some Chetopods, Chitons). 
A distinct method of further change consists in the approxi- 
mation to one another in the middle line of the two cords 
with their ganglia. This approximation may be slight, when 
it indicates an archaic condition (Peripatus, Serpula, &c.), 
or it may be so fully carried out that the ganglia, though 
rarely the cords themselves, are fused to one another in the 
middle line. 
Apus is remarkable, as the woodcut (fig. 2) shows, for the 
wide divergence of its nerve cords in the first part of their 
course behind the mouth, and in this disposition confirms the 
conclusion (to which the isolation of its archi-cerebrum 
leads) as to its nervous system being preserved in a primeval 
or archaic condition. ,' 
It appears that, just as there is a tendency in the primarily 
lateral nerve ganglia of bilaterally symmetrical animals to 
move towards one another and fuse in the middle line of the 
body, so also there is another and similar tendency for pri- 
marily distinct and isolated ganglia to travel either backwards 
or forwards (usually forwards) along the lateral cords, and 
