NERVOUS SYSTEM GASTEROPODA 



203 



The remaining three great divisions of (Gasteropoda, namely, 

 the Prosobranchiata, Opisthohranchiata, and I'uhnonata, may l»e 

 regarded as comprising two distinct types of nervons condition, 

 according as the loop formed by the two visceral nerve cords is 

 twisted over itself, forming a figure of 8, or continues straight 

 and uncrossed. In the former case, we get the condition known 

 as streptoneuroiis, \\\ the latter that as cuthynearovs} The 

 Euthyneura include the whole of the Opisthohranchiata " and 

 Pulmonata, the S'treptoneura all the Trosohranchiata. 



Fii.;. 101. — Xervous system of the Ampliineura : A, Proneomenia ; B, Neomemu ; C, 



<Ji.aetoderjiw : D, Chiton; c, cerebral ganglia; /, I, lateral cords; jk; posterior 



coiimiissure ; .«, suljliugual commissure or ring, with ganglia ; r, r, pedal cords. 

 (After Hulirecht.) 



The siniplest form of nervous system in the euthynenrous 

 ( i-asteropoda occurs in the Opisthohranchiata. The cerebral, 

 l)leural, and pedal ganglia tend to become concentrated in a ring- 

 like form, miited Ijy short commissures at the posterior end of 

 the pharynx. The visceral loop is in some cases long, and the 

 two or tliree visceral ganglia are then situated at its posterior 

 extremity. The nervous system of the rulmonata is of a, similar 

 type, the visceral loop l)cing often much shorter, and tending to 

 draw in towards the central group of ganglia. The tentacular 

 and optic nerves are, as usual, derived from the cerel)ral gang- 

 lion, with whicli also the otocysts are probably connected by 

 rathei- long nerves. A pair of buccal ganglia innervate the 

 buccal mass, and are united by connnissures with the right 

 aiul left cereliral ganglia. The osphradial nerve springs from one 



'' <7Tpeirr6s, twisted ; eiidiis, straight. 



- With the exception of Adaeon, which is streptoneurous (Rouvier, Comptes 

 J!r,iJi's\ cxvi. p. 68). 



