32 
W. J. Dakin 
directly (fig 22 os.h.n) to the root of the branchial nerve. It has been 
already shown that some of the fibres from the main branch enter the 
branchial nerve and it seems probable from sections that some of the 
fibres of the small nerve (fig. 22 os.h.n) enter the visceral ganglion in 
addition to those from the main branch. 
The osphradium is innervated by processes from ganglion cells lying 
along the branchial nerve and the branchial nerve root, and the fibres 
entering the branchial nerve and root swelling are from the visceral ganglion 
as well as from the cerebro-visceral connective. No separate fibre has 
been traced and there is therefore no evidence yet to show that the 
osphradium is innervated exclusively by the fibres from the visceral 
ganglion or the connectives in Mya. 
The nerve fibres innervating the osphradium reach the sense organ 
therefore by means of the branchial nerve or run directly from the 
visceral ganglion at the base of the branchial nerve. 
E nsis ensis. This species differs altogether from Mya. All 
the ganglia and most of the nerves are situated very close to the surface 
in this lamellibranch and are therefore readily seen without any dissection. 
The osphradia are only visible in sections. The visceral ganglia, which 
are fused, are situated on the anterior ventral edge of the posterior ad- 
ductor muscle. The cerebro-visceral connectives enter the cephalic or 
anterior ends of the ganglia (fig. 23 c.v.con), the posterior palliai nerves 
leave the posterior extremities and cross the adductor muscle, whilst 
the branchial nerves (fig. 23 hr.n) arise laterally, shghtly nearer the poste- 
rior margin and very close to the ventral surface of the fused ganglia. 
The branchial nerves rise immediately to the epithelium and then 
bend anteriorly, crossing the posterior retractor muscles and so reaching 
the gills. The osphradial epithelium is not very distinctly marked off 
by increased depth of the component cells but the characteristic cuti- 
cular margin and absence of cilia are as usuai well marked features. The 
osphradial nerve fibres arise from the branchial nerve and have an 
extremely short course. All the fibres of the cerebro-visceral connec- 
tive enter the visceral ganglion, passing within the ganglion cell cortex 
and fibres running into the branchial nerve must pass through the neu- 
ropil of the ganglion. 
Pholas dactylus. A species of PJiolas has been described by 
Pelseneer and certain fibres from the branchial nerve were again obser- 
ved to pass directly into the cerebro-visceral connective, some distance from 
the visceral ganglion. I have not been able to examine P. crispata the 
