Zoology 250 Phylogenetic Trees (2005)

(aplacophoran)
(polyplacophoran) 		PHYLUM MOLLUSCA

(Tree and traits from
Nielson 1995 p. 111, and
Brusca & Brusca 1990 p. 762)


(heterodont bivalve)
(scaphopods)

(prosobranch gastropod)
(nautiloid cephalopod) 

       ======2================== Aplacophora (shell-less, worm-like molluscs)
       |
<<=1===|    ====4=============== Polyplacophora (chitons)
       |    |
       ==3==|   ====6=========== Monoplacophora (primitive limpet-like molluscs)
            |   |
            =5==|         ===9== Gastropoda (snails, slugs, etc.)
                |    ==8==|
                |    |    ==10== Cephalopoda (nautiloids, cuttlefish, squids, octopus)
                ==7==|
 5="CONCHIFERA"      |    ==12== Bivalvia (mussels, clams, scallops, etc.)
                     ==11=|
                          ==13== Scaphopoda (tusk shells)

Back to Zool 250 tree for animal phyla or Protostomia

TRAITS SUPPORTING EACH CLADE (** plesiomorphic- a primitive state, not unique to clade):
1:
a) muscular, creeping ciliated foot**
b) mantle with calcareous spicules produced by single cells,
c) posterolateral mantle cavity with multiple bipectinate gills (ctenidia)
d) radula
e) larval/adult body with traces of eight segments
f) trochophore larvae**
g) coelom via schizocoely**
h) 'open' circulatory system (hemocoel) with pumping heart
i) circumpharyngeal nerve-ring (brain) with ladder-like ventrolateral nerves**
2= APLACOPHORA:
a) small, elongate, worm-like body
b) reduction/loss of foot & head
c) reduced number of ctenidia (two or fewer)
3:
a) large muscular foot
b) one or more dorsal, solid shell plates
c) 8 pair pedal 'retractor' muscles
d) mobile radular membrane & odontophore
4= POLYPLACOPHORA:
a) 8 unique shell plates (perforated by aesthetes)
b) mantle girdle
c) calcareous spicules from single cells**
5= CONCHIFERA:
a) 1 conical or coiled shell
b) 3-layered shell (periostracum, prismatic & nacreous calcareous layers)
c) crystalline style
6= MONOPLACOPHORA:
a) 8 pair pedal retractor muscles**
b) monopectinate ctenidia (5-6 pair)
7:
a) 2 pair pedal retractor muscles
b) 1 or 2 pair ctenidia
c) veliger larva (absent in Cephalopoda)
8:
a) coiled, tubular shell**
b) dorsoventrally expanded body with large, dorsal viscera
c) 1 pair pedal retractor muscles
9= GASTROPODA:
a) torsion
b) operculum attached to foot
c) 1 pair ctenidia**
10= CEPHALOPODA:
a) foot modified as siphon
b) shell with gas-filled chambers & siphuncle
c) ink gland
d) beak-like jaws
e) large, circumoral tentacles/arms
f) 'closed' circulatory system (vessels lined by epithelium)
g) image-forming eyes & large brain
11:
a) body modified for infaunal life
b) burrowing foot
c) paired, lateral larval shell plates
d) 1 pair ctenidia**
12= BIVALVIA:
a) radula lost
b) head lost
c) enlarged mantle cavity & ctenidia
d) mantle edge fused to shell
e) byssal gland
13= SCAPHOPODA:
a) 2 larval shell plates fuse into tubular adult shell
b) ctenidia lost
c) captacula

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Copyright (c) 2005 by A. Richard Palmer. All rights reserved.
(revised Jan. 5, 2005)