NERVOUS SYSTEM. 73 



mantle, huigs, genital glands aucl viscera. In Limax, Arion 

 and Oncidium the}' are placed below and more or less mask the 

 pedal ganglia. The stomato-gastric ganglia are small and usually 

 united b}- a transverse commissure ( absent in Testacella and 

 Glandina) ; they furnish nerves to the oesophagus, salivary glands 

 and lingual sheath. 



Pteropoda. The nervous system is analogous to that of the 

 gastropods, the thecosomata (Hyalea, Cleodora, etc.) having 

 atfinities with opisthobranchiata ; the gymnosomata (naked 

 species) with the pulmonifera. 



Scaphopoda. In this small group the nervous system resembles 

 that of the lamellibranchiates. There are four much dispersed 

 ganglionic centres: the buccal, pedal, anal, and stomato-gastric. 



Pelecypoda ( v, 69). The cerebral ganglia of the bivalve mol- 

 lusks are, as might be expected, small ; they are united by a loop 

 or commissure varying much in length, being long in Anodonta, 

 for instance, and so short in Mactra that the two ganglia may be 

 said to touch. They furnish nerves to the mouth, labial palpi 

 and anterior portions of the mantle. Commissures unite the 

 cerebral and pedal ganglia. These connectives are long in those 

 species like Mya and Modiola, in which the foot is distant from 

 the mouth; short in those in which the two organs are proximate, 

 as in Teredo; atrophied in those in which the foot is rudimentary, 

 as in Pecten and Ostrea. The pedal ganglia are two in number 

 but joined closely, or in Anodonta, soldered into a single mass. 



The commissures uniting the cerebral and branchial ganglia 

 are extremely long. The latter, two in number, are situated in 

 contact with the posterior adductor muscle. In Ostrea, Pecten, 

 Avicula and Mytilus they are united by a short connective, in 

 Anodonta, Mactra and Mya they are contiguous. In Teredo, 

 which has relatively very large branchiae, there is one on each 

 side of an accessory branchial ganglion. These ganglia furnish 

 not only two great trunks proceeding to the branchiae, but also 

 the nerves of the posterior adductor muscle, of the siphons, 

 heart, rectum, etc. A large number of small ganglionic swellings 

 occur in the course of these nerves. 



The splanchnic nerves proceed from the pedal or branchial 

 ganglia, and not from special ganglia as in the encephala. 



PhosphoreHcence. Pliny appears to have been the first natu- 

 ralist who noticed and recorded the fact that the Pholas, a 

 lamellibranch or bivalve mollusk, emits a phosphorescent light. 

 This faculty is shared by a number of pelagic gastropods, 

 colonies of which illuminate at night large portions of the surface 

 of the ocean, so that sometimes a ship appears to pass through 

 a sea of fire. The Pholas retains its luminosity after it has been 

 cut into fragments, and even after it has been dried, it will 



