Lesson 17 - Cephalopods

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Earth and Ocean Sciences
EOSC 326
Louise Longridge

Lesson 17 Learning Objectives 1 List and describe the engineering problems faced by the cephalopods in adapting to a pelagic mode of life 2 Describe how externally shelled cephalopods controlled their buoyancy 3 List and describe the numerous early nautiloid experiments related to pelagic adaptations For example the breviconeslongicones or actinoceratoidsendoceratoidsorthocerids 4 Compare and contrast ammonoids and nautiloids 5 Describe septal sutures and their function through geological time 6 Identify or sketch the septal sutures of nautiloids goniatites ceratites and ammonites Cephalopods Ammonoids and NautiloidsCephalopods are mollusks related to bivalves and gastropodsHighly mobile and fairly intelligent pelagic predators with good eyesightRoughly 10000 recognized species of ammonoids and nautiloids both groups are entirely marineEngineering problems Cephalopods with external shells had to solve some physical problems which were the driving forces behind their evolution 1 Buoyancy The shell is much denser than seawater and the tendency is to sink 2 Stability Stability is achieved by the separation of the centers of mass and buoyancy A round object has these two centers of mass together leading to instability 3 Hydrodynamic efficiency The round shape of the shell means there is high drag when moving under water 4 Hydrostatic pressure Pressure increases with greater depths the shell must withstand this pressure to stop implosion from happeningMorphology o Most externallyshelled cephalopods have a planispiral shell that coils on a single horizontal plane with the shell diameter increasing away from the coiling axisEach 360 revolution of the shell is called a whorl the very outside edge of each whorl is called a venter o A nautiloids shell has two internal partsThe living chamber houses all the soft parts which includes about 90 tentacles to help with food gathering and reproduction a hyponome used for jet propulsion a digestive system gills and reproductive organs The opening in the shell which the soft body protrudes is called the apertureThe phragmocone is made up of a series of gasfilled chambers or camerae These are separated from each other by hard pieces called septa A softtissue organ called the siphuncle runs through the center of the chambers connecting them to the mantle cavityThe siphuncle consists of blood vessels arteries nerves and other cells contained in an organic sheath It plays a key role in controlling buoyancy
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