MOLLUSGA—THE SHELL 



59 



the Gastropoda from bilaterally-symmetrical ancestors with symmetrical shells ; and 

 since (2) the FissureUidce nndoubtedly possess the most primitive organisation of all 

 the Gastroj)oda, and thus stand nearest to the racial form, and are moreover (3) 

 strikingly symmetrical in their organisation, it seems, at first sight, natural to 

 consider this symmetry a primitive feature. Certain peculiarities of the nervous 

 system, however, especially the crossing of the pleuro-visceral connectives, taken in 

 connection with other conditions explained more fully elsewhere, make it certain 

 that the cup-shaped shell of Fissurella is only secondarily symmetrical, i.e. that 

 Fissurella is descended from forms which possessed a spirally coiled shell. The same 

 is the case with the FatelUdce. 



The following important facts are in harmony with this conclusion : (1) the 

 young shell of Fissurella is asymmetrical and coiled, and it onlj' gradually assumes 



Pio. 50.— Shells of— A, Pleurotomaria ; B, Polytremaria ; C and E, Emarglnula ; D, Haliotis ; 

 F, Fissurella ; G ami H, stages in the development of the shell of Fissurella ; I, shell of the 

 twisted racial form of the Gastropoda with marginal cleft ; K, the same with apical aperture ; 

 L, shell of Lamellibranch ; M, shell of Dentalium, seen from the apical aperture. Tlie holes and 

 clefts of the shells are black ; o, mouth ; a, anus ; d, ctenidiuni. 



the symmetrical form (Fig. 59, G, H) ; (2) the apparently symmetrical shape of some 

 forms nearly related to Fissurella and Patella prove on closer inspection to be 

 somewhat asymmetrical, the apex especially being more or less excentric ; (3) 

 other forms nearly allied to Fissurella, such as Haliotis, Scissurella, and Pleuro- 

 tomaria, have spirally coiled shells (Fig. 59, A, B, C, D). 



In the FissureUidce, many Pleurotomariidcc, and the Haliotidcv, i.e. in the most 

 primitive Gastropods, pecidiar and noteworthy perforations of the shell occur, such 

 as are occasionally found in other divisions. Those perforations lie above the slit 

 in the mantle which is characteristic of this order {cf. p. 43), and they everywhere 

 establish communication between the mantle cavity and the exterior, esi)eciall}'' 

 needed when the mouth or edge of the shell is closely applied to the olyect on wjiich 

 the creature crawls. 



