156 



COMPARATIVE ANATOMY 



CHAP. 



apparently primitive shell. The latter is, however, only apparently primitive, as can 

 be proved systematically and ontogenetically. The forms most nearly related to 

 Fissurella, such as the primitive genus Pleurotomaria (Fig. 136 A), Polytremaria 

 (Fig. 136 B), and Scissurella, have spacious spirally coiled dextrally twisted shells. 

 In Haliotis (Fig. 136 D) the shell becomes flat and the coiling indistinct, as is also 

 the case to some extent in Emarginula (Fig. 136 C), till finally in Fissurella (Fig. 



FIG. 136. Shells of A, Pleurotomaria ; B, Polytremaria ; C, E, Emarginula ; D, Haliotis ; 

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

 the Gastropod racial form, with marginal cleft ; K, the same, with apical perforation ; 

 L, Lamellibranch shell ; M, shell of Dentalium, seen from the apical cleft. The shell clefts 

 and perforations are black, o, Mouth ; a, anus ; ct, ctenidium. 



136 F) it again secondarily becomes flattened or cup-shaped and symmetrical. Fis- 

 surella even passes ontogenetically through an Emarginula stage, in which the shell 

 is distinctly spirally coiled (Fig. 136 G, H). We may therefore conclude, with as 

 much certainty as is possible in morphological questions, that the outwardly sym- 

 metrical Fissurella descends from forms with high spirally coiled shells. Its return 

 to a flat symmetrical shell may have been determined, as in the Patellidce, Capulidce, 

 etc., by adaptation to certain biological conditions. 



11- 



The explanation given above seems to throw new light on many as yet unsolved 

 problems in the morphology of the Mollusca, such as the asymmetry of the pallial 

 complex in most Gastropoda. Many ' Diotocardia, all Monotocardia, all Opistho- 

 branchia, and all Pulmonata show marked asymmetry in the pallial complex. The 

 asymmetry consists principally in the absence of one gill, one osphradium, and one 

 nephridial aperture. The inner organisation also shows reflections of this asym- 

 metry in the nervous system, and the absence of one kidney and one auricle. On 

 closer inspection, it is found that it is the original left half of the pallial complex 



