VII 



NERVOUS SYSTEM — GASTEROPODA 



203 



The remaining three great divisions of Gasteropoda, namel}', 

 the Prosobranchiata, Opisthobranchiata, and Pulmonata, may be 

 regarded as comprising two distinct types of nervous 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 streptoneurous, in the latter that as eutliy7ieurous} The 

 Euthyneura include the whole of the Opisthobranchiata^ and 

 Pulmonata, the Streptoneura all the Prosobranchiata. 



I 1 , = /i I 



Fig. 101. — Nervous system of the Amphineura: A, Proiieomenia ; B, Neomenia; 

 C, Chaetoderma ; D, Chiton ; c, cerebral cranglia; /, /, lateral cords; pc, posterior 

 commissure ; s, sublingual commissure or ring, with ganglia ; v, v, pedal cords. 

 (After Hubrecht.) 



The simplest form of nervous system in the euthyneurous 

 Gasteropoda occurs in the Opisthobranchiata. The cerebral, 

 pleural, and pedal ganglia tend to become concentrated in a 

 ring-like form, united by short commissures at the posterior end 

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

 the two or three visceral ganglia are then situated at its posterior 

 extremity. The nervous system of the Pulmonata is of a similar 

 type, the visceral loop being 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 cerebral gang- 

 lion, with which also the octocysts are probably connected by 

 rather long nerves. A pair of buccal ganglia innervate the 

 buccal mass, and are united by commissures Avith the right and 



1 (rrpeTTTos, twisted ; eiiOvs, straisjht. 



2 AVith the exception of Actaeon, which is streptoneurous (Bouvier, Comptes 

 Bendus, cxvi. p. 68). 



