MOLLUSCS. 



255 



and then out through the excurrent opening. Singly these cilia are very weak, but 

 together they exercise a great deal of force. Many experiments have been tried by 

 cutting out a piece of the gill, placing it on a flat surface, and covering it with a 

 weight. The amount which will be moved by these minute lashes, under these circum- 

 stances, is almost beyond belief, the motion in one instance being six millimetres a 

 minute. 



The excretory organs are paired, and communicate internally with the cavity (peri- 

 cardium) surrounding the heart. The nervous system consists of three pairs of gang- 

 lia, a cerebral or supraoeso- 



phageal, a pedal, and a pa- 

 rieto-splanchnic pair. The 

 arrangement of these shows 

 many minor variations. 

 Normally, the first pair is 

 situate above the oesophagus, 

 but they may be brought be- 

 neath that tube, occupying 

 a position just outside the 

 pedal ganglia, which are, as 

 their name implies, situate 

 in the foot. The last pair 

 are placed just beneath the 

 posterior adductors. The 

 two latter pairs are con- 

 nected with the first by 

 double cords. Besides the 

 sense of touch, organs of 

 smell, hearing, and sight are 

 developed in most of the 

 group. The olfactory or- 

 gans are situated upon the 

 parieto - splanchnic ganglia, 

 the auditory organs near the 

 ganglia in the foot, while the 

 eyes are very variable in 

 position. The organs of 

 smell are merely patches of 

 elongated ejjidermal cells, 

 strictly homologous with 



.. ■ *i, 1 ^'^G- 264. — Diag''am of Anodonta; a^xnu^; 6, cerebro-visceral connective; 



Similar organs in other mol- 6r, cerebro-pleural ganglia; c, gill; rf, mantle; e, posterior adductor; ex, 



^ rpi 11 external lamella of inner gill; /, foot; g, genital opening; in, inner 



lUSCS. ine ears are small lamella of inner gill; /, visceral (parieto-splanclmie) ganglia; ;^, laWal 



sacs lined with cilia, each Palpi; -«, mouth; ;,, pedal ganglia; ,-, renal opening. 



containing a single otolith, which, by its vibration against the cilia, conveys the sensa- 

 tion to the nervous system. The eyes may be found either upon the edges of the 

 mantle or upon the tip of the siphon. In some forms like Spondylus, Pecten, 3factra, 

 etc., these eyes on the edge of the mantle are well developed, and, like those of On- 

 chidium, which will be mentioned in a succeeding page, are similar to those of verte- 

 brates, in that the nerve fibres penetrate the retinal body, and distribute themselves 



