28 
W. J. Diikin 
ganglia and commissure. The branchial nerves are as usuai situated 
close to the surface, in fact they arise from the ventral surface of the 
ganglion and pass immediately towards the epithelium. Furthermore 
the branchial nerves arise from points near the posterior extremities of 
the ganglia (fig. 17 hr.n) far removed from the cerebro-visceral connec- 
tives which are also somewhat deeply situated in the connective tissue. 
The posterior palliai nerve (fig. 17 f. fall) is again prominent. The os- 
phradia lie above the branchial nerves and extend some little distance 
on either side of them. They are both separate, the sensory epithelium 
being absent above the centre of the visceral ganglion. The epithelial 
cells are very definitely marked off from those of the non sensory area 
and are much deeper (fig. 17 os). They have a distinct margin and ciba 
are quite absent. The innervating nerve fibres leave the branchial nerve 
at various points along its course, the first arising at the junction of bran- 
chial nerve and visceral ganglion. All the fibres of the cerebro-visceral 
connectives enter the visceral ganglion and none pass therefore direct 
to the osphradium. 
Representatives of two suborders of the Eleutherojhabda have been 
considered, a third group, the Mytilacea has been examined by List who 
finds no evidence whatever of the osphradium being innervated by the 
cerebral ganglion. This brings us to the Synaptorhabda, the Eulamelli- 
branchiata -i- the Ostraeacea. 
Pinna nohilis. The visceral ganglia are fused forming one 
rather elongated ganglion of considerable size (fig. 18). The area inner- 
vated by this visceral ganglion is also large and hence the difference in 
size between cerebral and visceral ganglia. Three very prominent nerves 
leave the ganghon (which is situated on the posterior adductor muscle) 
on each side, namely, the cerebro-visceral connective {c.v.con), the 
branchial nerve (br.n) and the posterior paUial nerve (p. pali). Several 
smaller nerves also leave the ganglion, which are not figured. The 
osphradia are invisible except in sections, which are necessary to reveal 
their presence and position. They extend as in Area and Pectun- 
culus along and above (ventral) the branchial nerve (os), and are not 
continued over the visceral ganglion. 
The osphradia are innervated by fibres from the branchial nerves 
which lie very close to the epithelium. No nerve fibres reach the osphra- 
dium directly from the cerebro-visceral connectives. 
M actr a stultorum. This genus was the one originally exa- 
mined by Pelseneer. The species is not given in his first account of 
1889, the above species is figured in his second paper of 1891. I have 
