MOLLUSC A. 451 



branchiata1\ and the cephalic eyes as well as the pharynx. The pedal 

 ganglia supply the foot. The pedal nerves are sometimes connected by 

 a series of transverse commissures. Each pleural ganglion is continued 

 backwards into a nerve which unites posteriorly below, rarely above the 

 intestine, with its fellow, forming a visceral loop. This loop typically 

 carries a pair of visceral ganglia, and a posterior abdominal ganglion. 

 The pleuro-visceral system thus constituted supplies the viscera and the 

 walls of the body exclusive of the foot, and special ganglia are not infre- 

 quently developed on its nerves. In Fissurella and Haliotis (Gastropoda 

 Anisopleura), the pedal ganglia are replaced by long cords with an outer 

 coat of ganglion cells, extending down the foot and connected by trans- 

 verse fibrous commissures. The pleural ganglia are only incompletely 

 differentiated from the anterior ends of these cords. In Chiton and 

 its allies there are pleural as well as pedal cords both connected with 

 a cerebral ring, all alike invested with ganglion cells. The cords are 

 derived in Chiton, as are the ganglia in nearly all Mollusca, from the 

 epiblast. It is possible that these forms of nervous system represent 

 a primitive state, and that the concentration of ganglion cells into ganglia 

 is a later state. A pair of buccal ganglia are developed in the Glossophora 

 in connection with the buccal mass upon which they lie. They supply the 

 salivary glands, oesophagus and radula, and are connected to the cerebral 

 ganglia ; but it appears that in Fissurella, Haliotis and Turbo they are 

 really connected to the pleuro-pedal centres, their connective only travers- 

 ing the cerebral ganglia (Haller). 



Special sense-cells provided with immobile sense-hairs have been 

 found in the epidermis of Gastropoda and Lamellibranchiata. A special 

 patch of ciliated epithelium lies close to the base of the gills. A ganglion 

 in connection with a nerve derived from one of the visceral ganglia or 

 from the visceral loop underlies this patch. The whole constitutes the 

 osphradium, an organ which probably detects changes in the water 

 passing over the gills. The osphradium occurs in all groups of Mollusca 

 except Scaphopoda. An olfactory (?) pit is found near the eye in Cephalo- 

 poda. Cephalic eyes are absent in Scaphopoda and Lamellibranchiata, 

 and are rudimentary in Pteropoda if present. Eyes of a simple or complex 

 structure are scattered along the mantle-margin in many Lamellibranchiata. 

 In two instances (Pecten, Spondylus) these mantle-eyes bear a singular 

 resemblance to the Vertebrate eye : the visual rods are turned away from 

 the light, and there is a cellular lens derived however from the mesoblast. 

 A cellular lens and visual rods, turned as in Pecten and Spondylus, are 

 again met with in the dorsal eyes of certain species of the Gastropod 

 genus Onchidium. These three instances are, so far as is known, the sole 

 exceptions to the rule that the visual rods of Non-Vertebrates are directed 

 towards the light. A cellular lens also only recurs among Non-Vertebrates 



G g 2 



