236 THE LAMELLIBRANCHIA 
from one another, and remain so in most Protobranchia (Fig. 
214, v.g), in the Anomiidae, most species of Arca (Fig. 188, v.9), 
in Adacnarca and Philobrya, the majority of the Mytilidae, Avicula 
(Fig. 236), Ostraea, and certain Lucinidae (Montacuta). On 
the other hand, they are in juxtaposition in Voldia, Pectunculus, 
Limopsis, certain species of Arca, the Trigoniidae, Modiolaria, the 
Pectinidae, most Kulamellibranchia, and the Septibranchia. The 
visceral centres innervate the gills, the heart (by recurrent nerves 
passing round the posterior adductor muscle), the posterior part of 
the mantle, and the siphons. The anterior pallial nerves issuing 
from the cerebro-pleural centres and running along the borders of 
the mantle anastomose with nerves issuing from the visceral ganglia 
to form a complete pallial circle on either side. In some Eulamelli- 
branchia (Dreissensia, Pholadidae, and Teredinidae) there is a small 
but distinct ganglion mass in front of the visceral ganglia, and 
united to the two branches of the visceral commissure. In 
Dreissensia this accessory ganglion gives off several nerves, chiefly 
to the viscera. 
The Lamellibranchia have no differentiated stomato-gastric 
system; the median faces of the two branches of the visceral 
commissure give rise to nervous strands which pass to the 
alimentary canal. 
Tactile sensibility is specially localised in the most exposed 
parts of the body, that is to say, in the borders of the mantle along 
which run the circumpallial nerves formed by the anastomosis of the 
anterior pallial nerves from the cerebro-pleural ganglia with nervous 
trunks issuing from the visceral centre. The mantle borders very 
often bear sensory papillae, or more or less well developed tentacles 
throughout their extent, ¢.g. in Solenomya (Fig. 231, pa), Lepton, Pecten 
(Fig. 235, pa), and above all Lima: in this last genus the tentacles are 
long, contractile, and disposed in several rows. When the borders 
of the mantle are fused together at various points, these sensory 
papillae are localised at the posterior ends, at the place of entrance 
of the respiratory fluid, or at the margins of the siphons (Figs. 219, 
221), or round the two siphons to form a sort of tentacular crown, 
as may be seen, for example, in Cardium (Fig. 243, a.s, br.s), Tapes, 
Corbula, Poromya (Fig. 249, p.t). In some cases there are highly 
developed tentacles ; thus in Lepton and Galeomma there is a median 
azygos tentacle at the anterior end at the point of union of the two 
mantle lobes, and two symmetrical tentacles in the same situation 
in Solen. There are two symmetrical tentacles at the posterior end in 
Solenomya, and a single lateral tentacle on the right or left side in 
the Ledidae (Voldia, Fig. 230, st, Leda, and Malletia). The labial 
palps are not highly specialised tactile organs, and serve as acces- 
sory alimentary rather than sensory organs. 
At the origin of each great branchial nerve, close to the visceral 
