236 THE LAMELLIBRANCH1A 



from one another, and remain so in most Protobranchia (Fig. 

 "214, vi.g}, in the Anomiidae, most species of Area (Fig. 188, v.g), 

 in Adacnarca and Philobrya, the majority of the Mytilidae, Aricula 

 (Fig. 236), Ostraea, and certain Lucinidae (Montacuta). On 

 the other hand, they are in juxtaposition in Yoldia, Pectunculus, 

 Limopsis, certain species of Area, the Trigoniidae, Modiolaria, the 

 Pectinidae, most Eulamellibranchia, 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, e.g. in Solenomya (Fig. 2 31-, 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.. 2 19, 

 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 (Yoltlia, Fig. 230, s.t, 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 



