. Elements of comparative anatomy. Anatomy, Comparative. APPENDAGES OF MOLLUSCA. 3^7 all Dibranchiata. The arms of the Octopoda, like the similar ones in the Decapoda, are connected together at their bases by a web, ex- cepting the pair which are nearest to the sides of the funnel. This connecting membrane extends farther in some Octopoda ; sometimes over a few of the arms only (four in Tremoctopus), or over them all (Histioteuthis, more completely in Cirroteuthis), and is continued right up to the tips of the arms. The suckers are special structures found on the arms of the Cephalopoda; they
. Elements of comparative anatomy. Anatomy, Comparative. APPENDAGES OF MOLLUSCA. 3^7 all Dibranchiata. The arms of the Octopoda, like the similar ones in the Decapoda, are connected together at their bases by a web, ex- cepting the pair which are nearest to the sides of the funnel. This connecting membrane extends farther in some Octopoda ; sometimes over a few of the arms only (four in Tremoctopus), or over them all (Histioteuthis, more completely in Cirroteuthis), and is continued right up to the tips of the arms. The suckers are special structures found on the arms of the Cephalopoda; they generally beset the oral surface of the arms in two rows (one in Eledone), and not unfrequently they are carried on stalks. Their free edge has often a cuticular thickening which has the form of a chitinous rinsr, and is sometimes toothed. Where a particular tooth is largely developed the sucker disappears, and is replaced by a hook (Onychoteuthis). In many Cephalopoda certain of these arms are peculiarly altered by function- ing as copulatory organs ; even in Nautilus the tentacles perform this func- tion. It is not always the same arm that is thus metamorphosed; as a rule it is one of those which belong to the so-called ventral side of the animal. The mode of metamorphosis varies greatly in the different divisions; it may merely consist in the alteration of a part of the base of the arm (Sepia), or a more or less large number of the suckers may be altered, or the tip of the affected arm may be provided with a spoon-like hollow process (Octopus, Eledone). The highest grade of this adaptive metamorphosis is seen when the arm becomes greatly increased iu size, as well as different in organisation inter- nally (Argonauta, Tremoctopus). This " hectocotylised arm" is not developed, as are the others, by a process of free gemmation, but it is formed in a vesicle, from which it is not set loose till it is mature. The greatly-coiled flagelliform end of the arm (Fig. 173, y),
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectanatomycomparative
