2o6 



NERVOUS SYSTEM IN PELECYPODA 



by a commissure of varying length. Another pair of com- 

 missures unites them with the pedal ganglia, which are placed 

 at the base of the foot, and are usually ^-ery close together, 

 sometimes (as in Anodonta) becoming partially fused. The 



length of these commissures de- 

 pends upon the distance between 

 mouth and foot ; tlius they arc 

 very long in Myii and Modiuhi, 

 and very short in Fecten. In 

 cases where the foot is rudimen- 

 tary or Ijecomes aborted througli 

 disuse (e.g. Ostrea), tlie pedal 

 ganglia may dwindle or disappear 

 altogether. The visceral ganglia 

 are on the ventral side of the 

 posterior adductor muscle, beneath 

 the rectum, and innervate the 

 l)raiic]iiae, osphradia, and tlie 

 whole of the visceral sac. A pair 

 of cereltro - visceral commissures 

 traverses the base of the foot, 

 surrounding it witli a compara- 

 tively short loop (compare Fig. 

 106, cv.e), while a long connuis- 

 sm"e, which runs round the entire edge of tlie mantle, and sup- 

 plies branching nerves to the mantle border and siphons (Fig- 

 100, r.v.c), may also connect the visceral and cere1»ral ganglia. 



Cephalopoda. — In the Cephalopoda the concentration of 

 ganglia attains its maximum, and may perhaps be regarded as 

 approaching the point at which a definite brain may lie said to 

 exist. Another point of distinction is the formation (_)f special 

 small ganglia upon the nerve cords in different parts of the body. 

 In the Tetrabranciiiata (jVaufil'Us) the cerebral and pedal ganglia 

 form a liroad ring which surrounds the oesophagus, the former 

 giving out the optic nerves, with their special optic ganglion, and 

 a pair each of buccal and pharyngeal ganglia, the latter the 

 nerves of the arms and funnel. The A'isceral loop is still present 

 in the form of a separate band, which inner\^ates the branchiae, 



cerebro-pedal uiid }ileuro- pedal commissures can be recognised by sections of the mass 

 (Pelscneor, Comptcs llendus, cxi. p. '-'45). 



Fig. 105. — Nervous system of Pelecy- 

 poda : A, Teredo : B, Anodonta , 

 C, Prcten ; a, a, cerebral ganglia 

 h, pedal gau:j;lia ; e, visceral ganglia, 

 (At'ter Gegenbaur.) 



