MOLLUSCS. 



and then out thmiiiih IIr' fxcurri'iil (ipeiiiiig. lSiiii;l_v these eilia are very weak, hut 

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

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

 weiglit. Tlje amount which will be nioveil by these minute lashes, under these circum- 

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

 minute. 



Tlie excretory organs are pairetl, and couimunicate internally with the cavity (iieri- 

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

 lia, a cerebral or supraa'so- 

 phageal, a pedal, and a pa- 

 rieto-splanchnic ]iair. 'i'he 

 arrangement of these shows 

 many minor variations. 

 Normally, the first pair is 

 situate above the a>soj)hagus, 

 but they may be brought be- 

 neath that tube, occupying 

 a position just outside tlu' 

 pedal ganglia, which are, as 

 their name implies, situate 

 in the foot. The last pair 

 are jdaced just beneath the 

 ]iosterior a<lductors. The 

 two latter pairs arc con- 

 nected with the first by 

 double cords. Besides the 

 sense of touch, organs of 

 smell, hearing, and sight are 

 developed in most i>f the 

 group. The olfactory or- 

 gans are situated upon the 

 ])ariet() - s[>lanehiiie ganglia, 

 the auditory organs near the 

 ganglia in the foot, while the 

 eyes are very variable in 

 ]iosiiion. The organs of 

 smell are merely jiatches of 

 elongated epidi'rmal cells, 

 strictly honu)logous with 

 similar organs in other mol- 

 luscs. The ears are small 

 sacs lined with cilia, each 

 containing a single otolith, which, by its vil)ration against the cilia, conveys the sensa- 

 tion to the nervous system. The eyes may be f(3und either upon the edges of the 

 mantle or ii\)on the tip of the sijihon. In some forms like Spomfi//ii.% Pecteii, Jfactra, 

 etc., these eyes on the edge of the mantle .are w-ell developed, and, like those of Oii- 

 chtdiuDi, which will be mentioned in a succeeding page, are simil.ir to those of verte- 

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



Flo ot — Dug nni of 4nn<t nta t nniis I teiebio Mhcenil connective; 

 Ir loll I le 11 il g mglia gill / mantle e | ibtei lor aiiiincti>r; t-i', 

 e\ mil Un 111 (t ni lei giU t i t ft genital opening; in, inner 

 Inn 11 1 ol inner gill /, vivcii »1 (piiietc ■.plinchnic ganglia; Ip, labial 

 palp m nimth p pe lal ganglia ; rtnil opening 



