Zoology: Chapter 5 — Echinoderms & Invertebrate Chordates

5.1 Phylum Echinodermata: The Pentaradial Deuterostomes #

Echinoderms (sea stars, sea urchins, sea cucumbers, brittle stars, crinoids) are exclusively marine coelomate deuterostomes. While adults display secondary pentaradial symmetry (body organized in five radial sectors around an oral-aboral axis), their free-swimming ciliated larvae (bipinnaria, pluteus) are strictly bilaterally symmetrical.

5.1.1 The Water Vascular System and Ambulacral Mechanics #

The defining morphological synapomorphy of Echinodermata is the Water Vascular (Ambulacral) System—a complex hydraulic network derived from the embryonic coelom (left hydrocoel):

1. Seawater enters the system through an aboral, porous sieve plate: the madreporite.

2. Fluid flows down a calcium-reinforced stone canal into a circum-oral ring canal surrounding the mouth.

3. The ring canal gives off five radial canals extending down the ambulacral grooves of each arm.

4. Paired lateral canals branch off the radial canals, equipped with non-return valves, leading into hundreds of tube feet (podia).

5. Each tube foot consists of a rounded muscular bladder (ampulla) situated inside the body wall and an external tubular podium terminating in a suction disc.

       [WATER VASCULAR SYSTEM HYDRAULIC PATHWAY]
                      MADREPORITE (Aboral Sieve)
                                  │
                                  ▼
                             Stone Canal
                                  │
                                  ▼
                              Ring Canal (with Tiedemann's bodies & Polian vesicles)
                                  │
                                  ▼
                         5 Radial Canals (Ambulacra)
                                  │
                                  ▼
                            Lateral Canals (Valved)
                                  │
                                  ▼
                         AMPULLA  ◄──►  PODIUM (Suction Foot)

Biomechanical Stepping Cycle:

• Contraction of the circular muscles of the ampulla forces fluid into the podium, hydraulically extending the tube foot against the substrate.
• Chemical adhesives secreted by duo-gland epidermal systems cement the foot to the rock.
• Contraction of longitudinal muscles in the podium forces fluid back into the ampulla, shortening the foot and pulling the animal forward with immense hydraulic leverage.

5.1.2 Mutable Collagenous Tissues (Catch Collagen) #

Echinoderms possess a unique connective tissue property known as Mutable Collagenous Tissue (MCT) or “catch collagen”. Under direct neurological control via neurosecretory cells, the physical state of the collagenous dermis can reversibly transition from fluid and flexible to rock-hard within seconds:

• Mechanism: Calcium ions (Ca²⁺) released by nerve cells alter non-covalent cross-links between collagen fibrils.
• Function: Allows sea stars to maintain high muscular tension (e.g., prying open bivalve shells) for hours with zero metabolic ATP expenditure, or allows brittle stars to instantaneously self-amputate an arm (autotomy) to escape a predator.

5.2 Phylum Hemichordata: The Stomochord Link #

Hemichordates (acorn worms, e.g., Balanoglossus) bridge the anatomical gap between echinoderms and chordates:

• Three-Part Body Plan: Proboscis (Prosome) with muscular burrowing organ + Collar (Mesosome) + Trunk (Metasome) containing gill slits.
• The Stomochord: Historically mistaken for an ancestral notochord; it is a rigid, hollow diverticulum of the foregut extending into the proboscis, providing structural support for the excretory glomerulus.
• Tornaria Larva: The ciliated larva of acorn worms is so strikingly identical in morphology, ciliary bands, and deuterostome coelom formation to the bipinnaria larva of sea stars that J. Müller originally misidentified it as an echinoderm!

5.3 Invertebrate Chordates (Protochordata) #

Phylum Chordata unites animals possessing the four diagnostic hallmarks (see Chapter 6). Two subphyla lack vertebral columns:

🔬 Key Illustrated Scientific Figure

Full-Screen Zoom

Click to enlarge on this page

5.3.1 Subphylum Urochordata (Tunicata: Sea Squirts) #

Urochordates illustrate retrogressive metamorphosis:

• Ascidian Tadpole Larva: Possesses all chordate hallmarks: prominent dorsal notochord restricted to the tail (Urochordata = “tail cord”), dorsal hollow nerve cord, sensory statocyst/ocellus, and pharyngeal slits.
• Adult Metamorphosis: The larva settles on an anterior papilla. The tail, notochord, and nerve cord are completely resorbed; the dorsal ganglion degenerates into a simple neural gland.
• Adult Bauplan: The sessile adult is encased in a protective tunic composed of tunicin (an extraordinary animal cellulose identical to plant cellulose!). The body is modified into a colossal, perforated pharyngeal basket through which water is pumped from an incurrent branchial siphon to an excurrent atrial siphon, filtering microplankton with endostylar mucus sheets.

🔬 Key Illustrated Scientific Figure

Full-Screen Zoom

Click to enlarge on this page

5.3.2 Subphylum Cephalochordata (Amphioxus / Lancelet) #

Branchiostoma (Amphioxus) represents the quintessential archetype of the chordate body plan:

• Persistent Notochord: Extends along the entire length of the body, notably extending anterior to the brain into the snout (Cephalochordata = “head cord”), persisting throughout life. The notochord is composed of unique transverse, muscular lamellae that stiffen upon electrical stimulation.
• Metameric Myomeres: Longitudinal chevron-shaped (V-shaped) muscle blocks separated by myosepta, producing efficient lateral undulatory swimming.
• Endostyle and Wheel Organ: Ciliated oral hood and wheel organ sweep water into the mouth; an endostyle on the ventral pharyngeal floor secretes iodinated proteins and mucus, trapping food particles that are swept dorsally into the epipharyngeal groove.

5.4 Key Takeaways & Exam High-Yield Summary #

• Pentaradial Symmetry is Secondary: Echinoderm larvae are bilateral; pentaradial symmetry is a secondary adaptation to sessility and benthic feeding.
• Endostyle Homology: The protochordate endostyle is the direct evolutionary homologue of the vertebrate thyroid gland (both concentrate iodine and synthesize iodinated thyronines).
• Tunicin is Cellulose: Tunicates are the only metazoans capable of synthesizing cellulose, acquired via ancient horizontal gene transfer (HGT) of a cellulose synthase gene from bacteria.
• Amphioxus Excretion: Uses solenocyte-like cyrtopodocytes resembling platyhelminth flame cells, not true vertebrate nephrons.

5.5 Review & Self-Assessment Questions #

1. Explain the hydraulic mechanism that prevents backflow of water from the ambulacral tube foot into the radial canal during foot contraction.

• Answer: A delicate non-return one-way flap valve located inside the lateral canal automatically seals shut under positive hydrostatic pressure when the ampulla and podium contract, preventing backflow into the radial canal.

2. What anatomical changes occur during the retrogressive metamorphosis of an ascidian tadpole larva into a sessile adult?

• Answer: The tail, notochord, and dorsal hollow nerve cord are completely resorbed, the sensory ocellus and statocyst are lost, the dorsal nerve cord collapses into a single ganglion, and the pharynx expands into an immense filter-feeding branchial basket encased in a tunicin tunic.