. Cephalopoda. Cephalopoda. (128. 128 AlSvSl Ah An 5+ d. VARIATION OF THE TYPE OF DECAPODA Metamorphosis proceeds in three directions corresponding to the sub- orders Belemnoidea, =•' Teuthoidea and Sepioidea ( —49). The last suborder is quite independent (cf. Chapter 37), while the first two show closer relations which suggest a common ancestral form which we shall call "Metadecapus" in which the suckers and organs are arranged in a complicated manner. During the postembryonic development, some of them develop into hooks like in the Belemnoidea and Teuthoidea. Already inProtode
. Cephalopoda. Cephalopoda. (128. 128 AlSvSl Ah An 5+ d. VARIATION OF THE TYPE OF DECAPODA Metamorphosis proceeds in three directions corresponding to the sub- orders Belemnoidea, =•' Teuthoidea and Sepioidea ( —49). The last suborder is quite independent (cf. Chapter 37), while the first two show closer relations which suggest a common ancestral form which we shall call "Metadecapus" in which the suckers and organs are arranged in a complicated manner. During the postembryonic development, some of them develop into hooks like in the Belemnoidea and Teuthoidea. Already inProtodecapus () we assumed that the teeth at the distal edge of the horny ring func- tion like claws in grasping prey, particularly the median tooth which is the most prominent in size and position in many cases (Plate XII). Some cups may become further dif- ferentiated as suckers, others may develop into claws. The latter con- dition is more appropriate for the cups, which hold the captured prey, i. e. the larger and more proximal suckers, which are less able than the small terminal suckers to take hold of the prey rapidly ( smooth,hard- shelled Crustacea). The two types of weapon may also be used against different types of prey (soft- and hard-shelled). The recent species Gonatus fabricii () illus- trates the significance of the con- version of the suckers into hooks for division of labor. On the basis of this example, I assume that the conversion of suckers into hooks in the postembryonic stages of Metadecapus is restricted to the 2 inner rows of the arms and the 4 inner rows of the tentacles (which may also be 2 zig- zag rows), while the simple marginal rows of the arms, the zigzag-shaped marginal rows of the tentacles and the proximal and distal parts of all grasping arms retain the primitive dentition of the original suckers. The varied conditions in fossil and recent Decapodacan be best understood on the basis of such arrangements. For a better understanding of
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