The Lophotrochozoa - A Wealth of "Worms"

The Lophophorates - linked by the lophophore feeding apparatus
• Phoronida - The Horseshoe Worms
• Brachiopoda - The Lamp Shells
• Bryozoa - The Moss Animals

The Trochozoans - linked by the trochophore larva
• Mollusca - The Mollusks
• Annelida - The Segmented Worms
• Nemertea - The Ribbon Worms
• Sipuncula - The Peanut Worms
• Echiura - The Spoon Worms
• Pogonophora - Deep Sea Tube Worms

The Coelomate Bauplan:

• true plane of symmetry: bilateral

• three germ layers leading to true tissues
  • ectoderm (becomes the epidermis in the adult)
  • endoderm (becomes the gastrodermis in the adult)
  • mesoderm (derived from the endoderm)
• true organs and organ systems
  • integumentary system - YES
  • digestive system - YES
  • nervous system - YES
  • muscular system - YES
  • reproductive system - YES
  • excretory system - YES
  • circulatory system - YES
  • respiratory system - YES, IN MOST PHYLA

PHYLUM ANNELIDA - The Segmented Worms

Let's meet some very cute annelids.

Annelids all have

• typical protostome coelomate characters
• metamerism reflected in external and internal anatomy
• coelom serving as large, fluid-filled hydrostatic skeleton
• well-developed nervous system with a cephalic ganglion & ventral, ganglionated nerve cord
• a dorsal, closed circulatory system with several hearts
• appendages on each segment (even if simply bristles)
• metanephridia or protonephridia serving as excretory system
• larva a characteristic form called a TROCHOPHORE.

• Polychaeta - (poly = "many"; chaet = "bristle") Marine segmented worms
  • free-swimming or sedentary
  • distinct cephalization with complex sense organs
  • paddle-like appendages on each segment called PARAPODIA

• Oligochaeta - (oligo = "few"; chaet = "bristle") - Earthworms & their allies
  • free-living; freshwater or terrestrial; usually FOSSORIAL
  • reduced cephalization with reduced sense organs
  • small setae on each segment aid in locomotion through substrate
  • important detritivores in many ecosystems
• Hirudinea - (hirudo = "leech") Leeches
  • free-living predators/ectoparasites
  • coelom very reduced; body wall very muscular
  • salivary secretion "hirudin" is still sometimes used as an anticoagulant
  • many leeches are species specific, and many won't feed on a human

PHYLUM MOLLUSCA - The Mollusks

The Hypothetical Ancestral Mollusk (H.A.M.) had all the major mollusk characters in their most primitive form.

All Mollusks have

• typical protostome coelomate characters
• coelom reduced to a vestigial GONOCOEL (surrounding reproductive organs & a few other structures
• a haemocoel as the primary body cavity
• an open circulatory system
• viscera concentrated in a VISCERAL MASS
• a thick, epidermal-cuticular layer of skin called the MANTLE which secretes the SHELL
• large, well-defined, muscular "foot"
• buccal opening with a rasplike RADULA feeding structure
• large, complex metanephridia serving as excretory system
• primitive forms have a TROCHOPHORE larva; in more derived forms, the trochophore develops into a more complex VELIGER larva before metamorphosis into the adult form

• Polyplacophora - (poly = "many"; plac = "plate"; phor = "to bear") Chitons
  • all marine, benthic
  • of the forms we will consider, this is the closest to H.A.M.

• Gastropoda - (gastr = "stomach"; pod = "foot") - Slugs & snails
  • free-living; marine, freshwater or terrestrial
  • high degree of cephalization with well-developed sense organs
  • torsion
  • single shell, often spiral or conical
• Bivalvia - (bi = "two";valve = "shell"; ) - Clams, oysters, mussels, etc.
  • marine and freshwater
  • gill used for both respiration and gas exchange
  • all are suspension filter-feeders
• Cephalopoda - (cephalon = "head";pod = "foot"; ) - Chambered Nautilus, Squids, Octopus
  • exclusively marine
  • fast-swimming predators
  • camera eye is analogous to the vertebrate eye, and forms complex, color images like our own
  • comparatively intelligent; Octopus are able to solve problems!

Let's look at some Lovely Mollusks.

Both Annelids and Mollusks have a similar larval stage called a TROCHOPHORE:

...which is further testament to their evolutionary relationship. (Remember what we said about similar embryonic development in related groups!)

In the molluscs, the trochophore goes one step further and becomes a cute little VELIGER:

...before it undergoes further development to become whatever type of mollusk is its fate.