INVERTEBRATA. 333 



The pedal ganglia lie close together, just dorsal to the 

 foot, to whose muscles they send motor-fibres. 



The third pair, osphradial (or parieto-splanchnic) ganglia, 

 are also close together, just ventral to the posterior adductor, 

 which, as well as the posterior retractor and the inhalent 

 and exhalent apertures, they innervate. The cerebro- 

 osph radial commissures run from the cerebral close under 

 the skin until they reach the front end of the nephridium, 

 when they approach the middle line, and run close together, 

 diverging again to get round the posterior retractor. These 

 commissures are shown in section in fig. 168 (1) and (2), 

 and the osphradial ganglia in fig. 168 (3). 



Sense-organs are very few, as might be expected in an 

 animal shut up so completely in its shell. Doubtless the 

 epidermis is generally sensitive where it does not secrete a 

 thick shell, and the tentacles around the inhalent aperture 

 are more especially sensitive to touch and odour. A special 

 patch of sensory epithelium (olfactory?) occurs just over 

 the osphradial ganglia ; and in the foot there are a pair of 

 little round sacs, containing liquid and a solid particle 

 (otolith), which are generally assumed to be auditory organs. 

 Their nerves run to the cerebral ganglia, in the cerebro- 

 pedal commissures. There are no eyes. 



It would seem that each ganglion has an independent 

 power of reflex action; and it is very doubtful if any 

 process in such an animal can claim to be anything more 

 than a reflex action, simple or complex. Seeing how much 

 simpler this nervous system is than that of the unintelligent 

 dogfish, we cannot expect to find any trace here of memory 

 or the capacity for education, and it is probable that the 

 consciousness of Anodonta is of a most rudimentary 

 description. 



9. Reproduction. The gonads of Anodonta resemble 

 diffuse glands in structure, being tubular and quite con- 

 tinuous with their ducts. They are paired, and the ducts 

 open on either side close to the nephridiopore, into that 

 portion of the supra-branchial chamber which is enclosed 

 between the two lamellae of the inner gill (fig. 168 [2]). 

 Ovary and testis are exactly alike until examined micro- 



