372 PROFESSOR E. RAY LANKESTER,. 
dency is most obviously exhibited in the Mollusca, and in 
those animals affects more especially the part which has 
interest in connection with the morphology of Apus, namely, 
the adjacent preoral and postoral regions. 
It is now well established that the archi-cerebrum (the 
primitive prestomial ganglion) of Gasteropods may, as it 
were, attract to itself ganglia developed originally at a 
distance from it, which were connected to it by nerve-cords, 
and may be described as running up these cords to fuse with 
the archi-cerebrum, and thus to constitute a complex cerebral 
ganglion, containing altogether new morphological elements. 
Such a process of complicating the precesophageal ganglion 
pair is seen most obviously (to mention only one example) 
in the Nudibranchiata (Gegenbaur, English translation, 
p. 348), and a due appreciation of the possibilities of shifting 
and fusion of primarily distinct ganglia thus opened out 
affords the key to the very varied structure of the ganglion 
groups of the cephalous Mollusca. 
Whilst, then, we distinguish the original ganglion pair of 
the prestomial region as the archi-cerebrum—it is well to 
designate by a distinct term the composite ganglion, which 
may result from the fusion with it of other ganglia—it may 
be called a syn-cerebrum.! 
In the Chetopoda, the pra-cesophageal ganglion appears 
always to remain a pure archi-cerebrum. But in the 
Crustacea (and possibly all other Arthropoda, though there 
is a case to be considered for Peripatus and for the Hexa- 
poda and Myriapoda, on the supposition that their antennz 
are not the equivalents of Crustacean antennz but of the 
processes of the cephalic lobe of Chztopoda) the pre- 
cesophageal ganglion is a syn-cerebrum consisting of the 
archi-cerebrum and of the ganglion masses appropriate to 
-the first and second pair of appendages which were originally 
postoral, but have assumed a preoral position whilst carry- 
ing their garglion masses up to the archi-cerebrum to fuse 
with it. This is true of all Crustacea, excepting Apus and 
possibly some other Phyllopods, and in possessing a widely 
isolated and pure archi-cerebrum (woodcut, fig. 2, C) Apus 
stands forward in a very marked position. 
The only other case amongst adult Arthropods, in which it 
appears with certainty that the so-called cerebral ganglion 
is a pure archi-cerebrum, is that of Limulus.” Whilst some 
1 Tam aware of the objection which may be taken to the compounding 
in these terms of Greek prefixes with Latin substantives, but I see no 
alternative. 
2 Balfour has shown that in spiders the ganglion of the chelicere is in 
