. Cephalopoda. Cephalopoda. 75 relatives, if not the ancestors, of the true Nautilidae. The ammonites with modified shells probably had a different soft body, although no specific conclusions can be drawn. At any rate, neither Nautilus nor the related Ammonoidea are directly connected with the origin (type) of the Dibranchiata. However, Nautilus belongs to the central group of Nautiloidea, although its position there is much less certain than its relationship to the typical ammonites. Genera like Orthoceras, Cyrtoceras and Gyroceras certainly show at least the way in which the coiled Nautilus
. Cephalopoda. Cephalopoda. 75 relatives, if not the ancestors, of the true Nautilidae. The ammonites with modified shells probably had a different soft body, although no specific conclusions can be drawn. At any rate, neither Nautilus nor the related Ammonoidea are directly connected with the origin (type) of the Dibranchiata. However, Nautilus belongs to the central group of Nautiloidea, although its position there is much less certain than its relationship to the typical ammonites. Genera like Orthoceras, Cyrtoceras and Gyroceras certainly show at least the way in which the coiled Nautilus shell has evolved from the straight Orthocera s-like shell. Unfortunately, there is no direct information about the soft parts of these forms, and only indirectly, by a critical study of the correlation between shell form and soft body in Nautilus can some characters be interpreted as necessary adaptations or mechanical consequences of the shell form, while other characters have to be considered as an ancient inheritance or its metamor- phosis. The latter is especially true for characters shared by the Dibran- chiata which, in the last analysis, must have developed from the same root. These include peculiarities of organization not present in the Dibranchiata, but which must be assumed for their ancestors on morphological grounds ( typical correlations and embryological data). Some secondary characters of the Nautilus form can be excluded in this manner, first of all, for the straight O r t ho c e r a s-like ancestral forms of the Nautiloidea and also for the common ancestors of Tetrabranchiata and Di- branchiata (which may be identical or even more ancient than the former). This argumentation also eliminates as possible ancestors a number of fossil forms grouped around the genera Piloceras and Endoceras, since the structure of the siphuncle and other shell characters in these forms differ markedly from those of all other cephalopods. These two genera possibly (or probably)
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Keywords: ., bookcentury1900, bookcollectionbiodivers, booksubjectcephalopoda
