MOLLUSCA (V): DIVERSITY-Cephalopoda (Mollusca study images;
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a) includes largest (to 19 m), fastest swimming (to 25 kph), and most intelligent invertebrates; carnivorous (eat fish, crustacea, molluscs)
b) body plan modified for pelagic life; shell is reduced or absent in many; head and foot form tentacles and siphon
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) impressive eyes, balance organs (statocysts) and nervous system (largest brain in invertebrates, giant axons and stellate ganglia)
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) sexes separate; copulate with hectocotylus; unusual development without a discrete larval stage
i) 3 subclasses: I) Nautiloidea (1 genus); II) Ammonoidea (extinct); III) Coleoidea (Or. Decapoda, Or. Octopoda) (based on number of ctenidia and arms/tentacles, and on presence/absence of suckers)
PHYLUM SIPUNCULA: Peanut Worms(Sipuncula study images;
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Copyright (c) 2006 by A. Richard Palmer. All rights reserved.
(revised Mar. 2, 2006)