Introduction to Animal Morphology. 297 



ganglia : the upper are the cerebral ganglia; the lower 

 the pedal, behind which lie two parietosplanchnic 

 ganglia ; these are all joined by commissures (called 

 respectively commisstircB cerebraltSy pedalts, msceralis, 

 cerebro-pcdalisy and zn'scero-pedalis), and the ganglion 

 cells are often red or yellow. The epi- and hypo- 

 pharyngeals of each side may approximate, and may 

 even fuse, while the upper is often divided into two — 

 an upper, or brain, and a lower, or branchial ganglion, 

 or the epiphaiyngeals may unite and form a single- 

 lobed nerve (Heteropoda). As a rule, the higher the 

 animal the greater the degree of coalescence. 



Derived from the epipharyngeal, and on its primary 

 branches in some Opisthobranchs, there may be tentacular 

 and branchial as well as cerebral ganglia, and from the pedal 

 gangUa come off branches to the m. columellaris. The brain 

 ganglion gives oif branches to the tentacle, eye, olfactory or- 

 gan, ear, and penis. These commissural rings derived from 

 these ganglia in some Opisthobranchs surround the pharynx, 

 on the anterior of which are two buccal ganglia, united by a 

 commissure or fused into a bilobed ganglion in Littorina. 

 Each ganglion consists of unipolar, rarely bipolar, cells 

 in a neurilemma, which in Pulmonates (Zonites Algira) 

 has been shown to contain involuntary muscle fibres. 



The tentacles are absent in Chiton, two in most 

 Prosobranchs, or four in others — one pair often spe- 

 cialized for the support of the eyes [ommaiophores). 

 In these the nerves end in a ganglion, from which 

 pass fine filaments, ending in stiff surface bristles 

 (Trochus) ; they are covered by ciliated epithelium. 

 Between the tentacles of Phasianella and in front of 

 them is a median head lobe. A tentacle may spring 

 from the foot, as in Buccinum, Harpa, &c., in front or 



