22 MANUAL OF THE MOLLTJSCA. 



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sides and upper part of tlie ring represent tlie brain, and supply- 

 nerves to the eyes, tentacles, and mouth; other centres, con- 

 nected with the lower side of the oesophageal ring, send nerves 

 to the foot, viscera, and respiratory organ. In the bivalves, the 

 branchial centre is the most conspicuous, and is situated on the J 

 posterior adductor muscle. In the tunicaries, the corresponding i 

 nervous centre may be seen between the two orifices in the ^ 

 muscular tunic. This scattered condition of the nervous centres ', 

 is eminently characteristic of the entire sub-kingdom. • 



Organs of special sense. — Sight. The eyes are two in num- i, 

 ber, placed on the front or sides of the head ; sometimes they 

 are sessile , in others stalked, or placed on long pedicels (om- 

 matopJwra). The eyes of the cuttle-fishes resemble those of 

 fishes in their large size and complicated structm'e. Each con- : 

 sists of a strong fibrous globe (slerotic), transparent in front 

 (cornea)^ with the opposite internal surface (retina) covered by 

 a dark pigment which receives the rays of light. This chamber 

 is occupied by an aqueous humour, a crystalline lens, and a vi- 

 treous humour, as in the human eye. In the strombidce, the 

 eye is not less highly organised, but in most of the gasteropoda 

 it has a more simple structure, and perhaps only possesses sen- 

 sibility of light ^vithout the po\\^er of distinct vision. The 

 larval bivalves have also a pair of eyes in the normal position 

 (fig. 30) near the mouth ; but theii* development is not con- 

 tinued, and the adults are either eyeless, or possess merely ru- 

 dimentary organs of vision, in the form of black dots {ocelli) 

 along the margin of the mantle.* These supposed eyes have 

 been detected in a great many bivalves, but they are most con- 

 spicuous in the scallop, which has received the name of argm 

 from Poli, on this account (fig. 10). 



In the tunicaries similar ocelli are placed between the ten- 

 tacles which sm-round the orifices. 



* " Each possesses a cornea, lens, choroid and nerve ; they are, without 

 ■ Garner. 



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