BILATERIAN ANIMALS (modified from WWW Tree of Life; traits & tree after Ruppert & Barnes 1994 p. 1052).

               =========================== Platyhelminthes (flatworms, tapeworms, etc.)
               |
               |  ====(branch uncertain)== pseudocoelomates (nematodes, rotifers, etc.)
     ====P=====|  | 
     |         |  |      ================= Nemertea (ribbon worms)
     |         |  |      |
     |         ===|      |        ======== Sipuncula (peanut worms)
     |            |      |   ==M==|
     |            |      |   |    ================================= TO MOLLUSCA ===>
     |            ===C===|   |
<<=B=|                   |   |      ====== Echiura (proboscis worms)
     |                   |   |      |
     | B- BILATERIA      =S?=|   ===|  === Pogonophora (beard worms, vent worms)
     |                       |   |  ===|
     | P- PROTOSTOMIA        =S?=|     === Annelida (segmented worms)
     |                           |
     | C- COELOMATE PROTOSTOME   |    ==== Onychophora (velvet worms)
     |                           =====|
     | S- SEGMENTATION                ==== Arthropoda (insects, spiders, crabs, etc.)
     |
     ===================================== DEUTEROSTOMIA (echinoderms, chordates, etc.)

TRAITS SUPPORTING BRANCH M: a) "molluscan cross" at 64-cell stage b) scraping buccal organ * c) muscular creeping foot * * traits b & c are only present in Sipuncula larvae

       ======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)

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: a) small, elongate, worm-like body b) reduction/loss of foot & head c) reduced # 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: 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: a) 8 pair pedal retractor muscles** b) monopectinate ctenidia (5-6 pr.)

7: a) 2 pair pedal retractor muscles b) 1 or 2 pr. 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: a) torsion b) operculum attached to foot c) 1 pr. ctenidia** 10: 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) burrowing foot b) paired, lateral larval shell plates c) 1 pr. ctenidia** 12: a) radula lost b) head lost c) enlarged mantle cavity & ctenidia d) mantle edge fused to shell e) byssal gland 13: a) 2 larval shell plates fuse into tubular adult shell b) ctenidia lost c) captacula

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