THE THREE MODES OF l'i:\ ELOPMEN I 



- conclusively the comparatively small influence of natural b< 



• during the acme 



An :n favor of 1 1 1 • - views here advocated i- t«i be found 



in the cycle formed by the modes of development The more ancient ami sim- 



pler forma of Cephalopoda, like Orthoceras and most of the earlier forms of 



Nautiloidea, had ■ direct mode of development, during which the individual 



i through certain well marked changes ; but thes< s numerous and 



tlier as in types with more complicated development. The 



and adults of the Bame individual among Btraight radical shells differed 



iratively little in form and ornamentation The siphon in Piloceras, and 



in some species of Endoceras, for example, was comparatively little changed in 



adults from what it had been in the young, and Cyrtocerina as described above, 



bly remained completely macrosiphonitic throughout life. 



When closer coiling was introduced and more concentrated development 



occurred, a< in the higher Nautiloidea and Ammonoidea, we found, as one of 



-. the omis some hereditary As we have said above, 



tin 1 first air-chamber disappeared in Ammonoidea, having become fused with 



the protoconch, wbi acquired through tlii> earlier inheritance a ten- 



- -lndl. and in consequent t' tin-; fusion the 



pbonula w irried bark and appeared earlier in the life of the dull 



and animal. 



The naepionic or true larval stages, as we have said above, became more 



rated in the angustisellate young of the Lytoceratinsa and Ammonitinae, 



and tl ling or nealogi ?ere introduced with a profusion of orna- 



- and increased complications in the outlines of the Butures, curves of the 



structure of siphon, increased involution, and changes <•! structure and 



form in 1 1 1 • - whorls, which multiplied the metamorphoses and made 1 1 » « - different 



uwtli i e distinct than in the GoniatitinsB and Nautiloidea. 



jil<'\ mode of development of the normal acmic forms, due to the 

 •i i.f these new characterist ved in most of the 



Ammonitime, even without breaking down the whorls to examine the y< 



I and pilae on the ex| I sides of the whorls show tlii- in 



with sufficiently discoidal shells, 

 ms, and especially the degenerate uncoiled s] 

 ■ .in to the direct mode <>t development, and 

 •id number "I structural changes. These suppressions be- 

 \c described them o well marked in all the Btrniglil 



ninot fail to notice the fact The Binooth, 

 ■lightly compressed whorl retained nearly the same form throughout life, 1 1 1 « - 

 sntun nitive nu and the mat 



lively simple even in adults, 

 phenomen oup. Thus in 1 1 * « - the 



had the 

 >pment while the descendenl * »< » 1 1 ■ tin - 



transform 



