Nervous System of the Razor-shell Clam 315 



the foot or the collar, but only by stimulation, usually when in 

 the proper position, of the posterior or ventral mantle region. 

 The habits of the animal are such that these regions are most 

 likely to give warning of the presence of enemies. 



If instead of applying reasonably strong and repeated stimuH, 

 such as would be caused by stroking or pricking, very light and 

 short stimuli are given, such as may be given by barely touching a 

 tentacle with the side of a needle, a different result may be obtained. 

 With a specimen lying in a dish of sea-water it is possible, by 

 repeated slight stimuh, to cause the siphons or the foot and collar 

 to be withdrawn without visibly affecting other portions. The 

 foot and collar are so intimately associated, touching each other 

 as they do, that it is very hard to cause the retraction of one with- 

 out the other, but it is possible to cause a marked change in one 

 without appreciably changing the other. 



Before considering reactions further it is desirable to give atten- 

 tion to the nervous system. 



The three pairs of gangha that are usually present in lamelli- 

 branchs are all well developed, but there are no other definite 

 ganglia. There seem to be a few scattered ganglion cells about 

 the branchial nerves and a few others in sensitive portions of the 

 mantle, but on the whole the nerves, commissures and connectives 

 are remarkably free from ganglion cells. Although small ganglia 

 are reported to be present on the cerebro-visceral connectives of 

 Solon,^ a very closely related form, I find no trace of such ganglia 

 in Ensis, either in the dissections of mature individuals or in the 

 serial sections of individuals about two centimeters long. 



The cerebral gangha, Figs, i and 2, eg, lie directly ventral to the 

 anterior foot muscles and anterior to the mouth. They are far 

 apart and are connected by a narrow commissure, Fig. 2, cc, in 

 which ganglion cells do not seem to be present. Each cerebral 

 ganglion is joined to the corresponding visceral and pedal ganglion 

 by connectives, Figs, i and 2, eve and cpc, in neither of which 

 are ganghon cells abundant. Posteriorly each cerebral ganghon 

 sends a nerve to supply the labial palps of the same side, Fig. 2, 



' Lankester's A Treatise on Zoology, part 5, MoUusca (Pelseneer); Cambridge Natural History, 

 MoUusca. 



