238 AN INTRODUCTION TO THE STUDY OF FOSSILS 



The osphradium, with function possibly somewhat of the 

 nature of smell, is a ciliated region specialized by an accumu- 

 lation of sensory cells, and situated at the outer edge of the gill, 

 where the current of water supplying the gill must pass over it. 

 Otocysts, the organs of hearing, are located at the base of the 

 tentacles; they are hollow sacs containing a liquid and some 

 auditory concretions and with ciliated walls possessing sensory 

 cells. The organs of sight are two small sessile eyes upon the 

 outer edge of the tentacles ; they are provided with a protect- 

 ing cornea, a light-focusing lens, and a retina with pigmented 

 and sensory cells. 



Two of the more important muscular areas are the foot and 

 the columellar muscle. The/00/ is a powerful mass of muscular 



tissue with a fiat lower surface 

 adapted to creeping. Successive 

 waves of movement pass over this 

 surface wherein successive points 

 are raised, passed forward and again 

 planted and thus effect the creep- 

 ing movement of the whole foot, 

 as in walking a man raises one foot, 

 pressing the other at the same time 

 against a resistant surface. The 

 animal is held in its shell by the 

 contraction of the columellar muscle 

 which extends from the concave 

 right side of the body to the colu- 

 mella to which it clings, its place of 

 attachment moving spirally forward 

 Fig. 103. -The shell of Busycvn ^^.-^.j^ ^j^^ growth of the shell. 

 canahculalus, with a portion " 



broken away showing the method The shell is formed by the mantle 

 of coiling, a apex of spire ^g ^^ Pelecypoda (p. 214) ; the sur- 



the position of the protoconch j r ^i 



when preserved; c/>., aperture face irregularities of the shell merely 

 of shell; CO columella; i.lp reflect Corresponding surface irregu- 



inner lip; o.lp., outer lip. (A 10 <-■ 



small individual X i) larities of the mantle. The closely 



