. Cephalopoda. Cephalopoda. FIGURE 55. Diagrams of the derivation of Teuthoidea. Shown are 4 young Decapoda. a— Protodecapus; b — transitional stage; c — Prototeuthis; d — Metateuthis (see below). The reduction of the phragmocone results in an expansion of the muscular mantle and viscera toward the posterior end. The rostrum is still absent in these stages, also if it develops later (see p. 108). Because of the reduction of the phragmocone, the fins are situated close to the posterior end. The relatively narrow proostracum at the transition 139 to the cone brings the two fins closer to the dor
. Cephalopoda. Cephalopoda. FIGURE 55. Diagrams of the derivation of Teuthoidea. Shown are 4 young Decapoda. a— Protodecapus; b — transitional stage; c — Prototeuthis; d — Metateuthis (see below). The reduction of the phragmocone results in an expansion of the muscular mantle and viscera toward the posterior end. The rostrum is still absent in these stages, also if it develops later (see p. 108). Because of the reduction of the phragmocone, the fins are situated close to the posterior end. The relatively narrow proostracum at the transition 139 to the cone brings the two fins closer to the dorsal median line without uniting them in the middle. They are also still very small in relation to recent fins, although they have the same rounded-triangular form with an anterior indentation, which forms an "earlobe" (Figure 53). The articulation with the body, i. e. with the conus part of the shell, resembles that of Protodecapus (p. 114), but the capsule of the articulation is completely separated from the shell sac (Figure 56). A special character of all Teuthoidea is the form of the mantle margin, which forms sharp, markedly projecting angles on each side of the funnel incision in contrast to the rounded angles of most Sepioidea. The arm apparatus of Prototeuthis shows the external characters inherited from M e t a d e c a p u s (p. 127), at least in the juvenile stages. The arms and tentacles bear, respectively, 4 and 8 rows of suckers. Of these, the two median rows on the arms and part of the 4 median rows on the marginal part of the tentacle club develop into hooked suckers and true hooks (p. 133). Swimming and protective margins are developed as in Proto- decapus and M e t a d ec a p u s (Figure 51). The tentacle pockets have developed further: in the Teuthoidea, they penetrate posteriorly between the muscular organs of the head, and separate them from each other. This change, however, may have been only slightly indicated in Prototeuthis. The basal p
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Keywords: ., bookcentury1900, bookcollectionbiodivers, booksubjectcephalopoda
