MOLLUSCA (V): DIVERSITY-Cephalopoda (Mollusca study images;
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b) body plan modified for pelagic life; shell is reduced or absent in many; head and foot form arms and funnel
c) extinct forms had well-developed shells; gas-filled chambers (mainly nitrogen, some carbon dioxide) provided buoyancy
d) mantle is muscular and mantle cavity is large; jet-propulsion is achieved via antagonism of circular and radial mantle muscles
e) remarkable ability to change color rapidly using chromatophores
f) 2 pair (Nautilus only) or 1 pair (all others) of ctenidia lack cilia!
g) have a true 'closed' circulatory system lined by endothelium, with accessory hearts for gills
h) impressive eyes, balance organs (statocysts) and nervous system (largest brain in invertebrates, giant axons and stellate ganglia)
i) sexes separate; males copulate with hectocotylus; unusual direct development in benthic egg cases with no discrete larval stage
j) 3 subclasses: I) Nautiloidea (1 genus); II) Ammonoidea (extinct); III) Coleoidea (Or. Decapoda, Or. Octopoda)
Trait | Nautiloidea | Ammonoidea | Coleoidea, Decapoda | Coleoidea, Octopoda |
---|---|---|---|---|
common names | nautiloids | ammonites | squid, cuttlefish | octopuses |
shell | large, external; simple sutures; central siphuncle |
large, external; complex sutures; marginal siphuncle |
reduced to cuttlebone or pen | absent |
ctenidia | 2 pair, bipectinate | ?? (extinct) | 1 pair, bipectinate | 1 pair, bipectinate |
arms | 38, suckerless | ?? (extinct) | 8, with suckers | 8, with suckers |
tentacles | absent | ?? (extinct) | 2, with suckers | absent |
Copyright (c) 2017 by A. Richard Palmer. All rights reserved.
(revised Dec. 19, 2016)