LECTURE 10 - Spiralia: Mollusca part 2 Flashcards
1
Q
What are Cephalopoda?
A
squid and octopus
2
Q
Describe cephalopoda
A
- Head and foot fused; foot elaborated into arms or tentacles
- Shell reduced or absent (except Nautilus)
- Mantle cavity encloses visceral mass and has muscular walls
- Inhalant pore is opening in mantle; exhalent siphon (funnel) formed from foot
- Water current generated by muscular contraction of funnel or mantle wall
- Pair of unciliated gills in mantle cavity for respiration
- Closed circulation with heart
- Nervous system with giant axons and fused ganglia forming brain
- Advanced image-forming eyes
- The radula lies on the floor of the buccal cavity, with a chitinous beak secreted by buccal tissue surrounding it
3
Q
Describe the suckered arm.
A
- The arms or tentacles of decapods and octopods are armed with suckers
- These are suction devices which adhere to hard surfaces in the same way that limpets cling to rocks (but without glue-like mucus adhesion)
- The lowest pressure that can be produced is limited by cavitation; the cavitation limit is 100-200 kPa
- Ambient pressure depends on depth, so the greatest pressure that can be sustained may be limited by the musculature of the sucker
- Decapods have stalked suckers consisting of a rigid cylinder, a muscular piston that fits into this cylinder, and a thin, tough stalk that connects the piston to the arm or tentacle club
- Pulling on the stalk pulls the piston back against the resistance of the enclosed water, creating a powerful suction
- The sessile suckers of octopods, in contrast, are flexible, muscular cups
- The musculature of the wall of the cup generates an expansive force that decreases the pressure of the enclosed water
4
Q
How does the octopus’ brain learn?
A
- the octopus nervous system has three main parts:
- The optic lobes
- The arm lobes
- The central brain
- The brain surrounds the oesophagus; it contains about 50m neurons encased in a cartilaginous box
- It represents a condensation of ganglia by shortening the connectives between them and thereby increasing the speed of computation
- This large, complex brain scales somewhere between a lizard and a pigeon
- It makes a wide variety of new kinds of behaviour possible, including observational learning and imitation
5
Q
What is the Nautilus?
A
- A sole survivor
- Nautilus includes a few closely-related species similar in structure to the large Palaeozoic radiations of nautiloids and ammonoids
- Animal inhabits a coiled chambered shell into which it can completely retract
- Many tentacles, without suckers, convey prey to beak and radula
- Eyes open. operate as pinhole camera; two pairs of ciliated sensory tentacles near eyes
- Locomotion by jet propulsion using funnel
- Restricted to Indo-Pacific tropics at moderate depths (100-500m) on fringing reefs (the shell implodes at 800m)
- Predators on crustaceans; also scavengers
6
Q
What are Polyplacophora (creeping scrapers)?
A
- Chitons
- Untorted, bilaterally symmetrical molluscs with shell of eight dorsal plates arranged in a row, sometimes obscured or concealed by overgrowth of mantle
- Creeping foot bordered by pallial groove bearing gills
- Distinct head, lacking eyes and tentacles; stout radula; coiled intestine with terminal anus
- Feed by scraping algae from rocks, passed to mouth in mucus stream generated by cilia
- Nervous system consisting mainly of circumenteric ring that gives off paired longitudinal, lateral and pedal nerve cords
- Fertilization external except in brooding species, where young are held in pallial groove of female
- Spiral determinate cleavage with gastrulation by invagination leading to trochophore larva
7
Q
Why do we say that Polyplacophora have metal teeth?
A
- Chitons mineralize their teeth not only with calcium phosphate but also with iron oxides, including magnetite
8
Q
What is considered a field of eyes in Chitons?
A
- The carapace
- The paltes covering the dorsal surface of Acanthopleura bear hundreds of small eyes
- Each eye has a lens made of aragonite and is capable of forming an image, for example of an approaching predator
9
Q
What are Aplacophora?
A
- Untorted, bilaterally symmetrical vermiform molluscs without head, mantle, foot, shell or nephridia
- Body covered with calcareous spicules
- Longitudinal pedal groove runs ventrally most of the length of the body, with floor bearing one or more pedal folds; this may correspond with foot of other molluscs
- Mouth a subterminal slit, highly expansible; straight gut leads to posterior anus
- Radula usually present, sometimes absent; mounted directly on foregut epithelium
- Caudofoveates are deposit feeders; solenogasters eat cnidarians
10
Q
Describe the anatomy of Aplacophora
A
- Capacious anal chamber houses pair of gills or ciliate respiratory epithelia
- Nervous system consists of a large cerebral ganglion anteriorly above the foregut, from which paired nerve cords ending in ganglia extend anteriorly and posteriorly
- Body cavity with scattered connective tissue, no definite coelom
- Circulatory system very open and diffuse, with blood soaking the interior spaces
- Dioecious or hermaphroditic
- Spiral cleavage with gastrulation by invagination or ingression leading to a trochophore larva
11
Q
What are Monoplacophora?
A
- Untorted univalve biletrally symmetrical molluscs with serialle repeated external gills, nephridia, ventricles, foot retractor muscles and nerve branches
- Mantle encircles body as a fold of body wall and underlies limpet-like shell
- Broad ventral foot
- Definite head, without eyes or tentacles
- Ventral mouth with radula in pharynx
- Nervous system consists of a circumenteric ring with large ceregral ganglion from which issue paried lateral and pedal nerve cords
- Open circulatory system with extensive blood sinuses surrounding viscera
- Dioecious with two pairs of gonads, each with short gonoduct discharging into adjacent nephridium
12
Q
Is there a specific body form characteristic to molluscs?
A
No, there’s an extreme diversity of body form in molluscs
