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and direct cause of the correlated changes of structure which dis- 

 tinguish the different orders and suborders, and often of the 

 exceptional genera and species. 



We will mention but one of these exceptional cases, in some 

 respects the most pertinent — the existing Argonauta, or paper 

 nautilus (Fig. 2, p. 357). Here a thin shell secreted by the mantle, 

 by the edge of the mantle, and by the two pairs of long dorsal 

 arms, encloses completely the animal of the female alone, the male 

 being naked. As a sexual organ for the protection of the eggs ; as 

 an adolescent and adult structure, originating at a late stage in the 

 life of the individual, and not in the shell gland of the embryo ; and 

 in its microscopical structure — it is not a true shell, or similar to any 

 true shell among Cephalopoda. Still, in form and position, and 

 as built in part by the mantle, it is analogous to a true shell, and 

 has in part also the functions of a true external shell, and ought 

 therefore to support or refute the hypothesis maintained above. It 

 belongs to a swimming animal, and should therefore have the 

 hyponomic sinus in the aperture and striae of growth as in Nauti- 

 loidea; and these it certainly has. Compare the side view of 

 Nautilus umbilicatus (p. 354, Fig. 1), with the Argonauta and it will 

 be seen that the lines of growth agree in both and that both pos- 

 sess the hyponomic sinus on the outer side. One can appeal to 

 this example as a most convincing exception to prove the rule that 

 the shell is a true index of the most remarkable adaptive structures, 

 and, among the fossils, can give us exact information of important 

 similarities or differences in structure and habits. 



The efforts of the Orthoceratite to adapt itself fully to the 

 requirements of a mixed habitat of swimming and crawling gave 

 rise to the Nautiloidea; the efforts of the same type to become 

 completely a littoral crawler evolved the Ammonoidea. The suc- 

 cessive forms of the Belemnoidea arose in the same way. But here 

 the ground-swimming habitat and complete fitness for that was the 

 object. The Sepioidea, on the other hand, represent the highest 

 aims as well as the highest attainments of the Cephalopods in their 

 evolution into surface-swimming and rapacious forms. We cannot 

 seriously imagine these changes to have resulted from intelligent 

 effort ; but we can with Lamarck and Cope picture them as due to 

 efforts on the part of the animal to take up new quarters in its en- 

 vironment and thus acquire habits and structures suitable to the 



