Biology 2nd Paper (Zoology) · Classes 11-12 · Chapter 1

Nonchordata: Mollusca to Echinodermata.

Put a snail, an earthworm, a cockroach and a sea star side by side and you will struggle to find a likeness. All four are nonchordates, yet they look nothing alike. The key to telling them apart is a few special features of each phylum: foot, ring, joint and spine.

1. Mollusca: a soft body in a hard shell

Mollusca comes from the Latin molluscus, meaning soft. Linnaeus named the phylum in 1758, and 84,977 living species are known. In numbers it stands just after Arthropoda, which makes it the second largest phylum of the animal kingdom. Its members are unique shell-bearing nonchordates: mussels, snails, octopus, Sepia and Loligo.

The body is soft, fleshy and unsegmented. They are coelomate, mostly bilaterally symmetrical (Gastropoda excepted), with a distinct head. The body is covered by a thin sheet called the mantle, and the calcareous shell is built from material the mantle secretes. The body cavity is very small and has become a haemocoel. The thick skin of the ventral side has turned into a broad, muscular foot.

The gut is coiled, sometimes U-shaped. The mouth cavity holds a chitinous rasping tongue, the radula, which Bivalvia lack. Breathing is by gills (ctenidia), lungs, both, or the mantle; the blood carries haemocyanin and amoebocyte cells. The heart lies dorsally. Both blood vessels and a haemocoel are present, so the circulatory system is semi-open.

Most Mollusca live in salt sea water; some live in fresh water, on land and in burrows. Examples (Figure 1.20): Uroteuthis duvaucelli (squid), Lamellidens marginalis (freshwater mussel), Pinctada vulgaris (pearl oyster), Pila globosa (apple snail), Octopus vulgaris (octopus).

2. Annelida: a row of rings

Annelida comes from the Latin annelus (little ring) and ida (form). Lamarck named the phylum in 1809, and 17,388 living species are known. They are of organ-system grade, bilaterally symmetrical and triploblastic, with a more centralised nervous system and a more complex circulatory system than the earlier phyla. Polychaete annelids are mostly marine; earthworms and leeches live in fresh water or on land, and many leech species suck blood.

The body is long, cylindrical and bilaterally symmetrical, covered by a thin cuticle secreted by the epithelium, and has a true coelom. Here is true segmentation: many alike, ring-like segments. The locomotor organs are chitinous setae or muscular parapodia. In almost every segment a coiled tubule, the nephridium, works as the main excretory organ.

The circulatory system is closed and the blood is red, with haemoglobin, haemerythrin or chlorocruorin dissolved in the plasma. The gut is tubular and complete. Indirect development produces a free-swimming trochophore larva. Many are free-living, a few are parasites. Examples (Figure 1.22): Metaphire posthuma (earthworm), Arenicola marina (sand worm), Amphitrite ornata (ornate worm), Pontobdella muricata (marine leech), Tubifex tubifex (blood worm), Hirudinaria medicinalis (leech).

3. Arthropoda: jointed feet

Arthropoda comes from the Greek arthron (joint) and podos (foot). Von Siebold named the phylum in 1848, and 1,257,040 living species are known. This is the largest phylum of the animal kingdom: three-quarters of all recorded animals of the earth belong to it, and its social life is unique.

The body has jointed appendages, is bilaterally symmetrical, segmented, and divided into tagmata: head, thorax and abdomen. The head bears one or two pairs of antennae and usually a pair of compound eyes. The exoskeleton is made of chitin and is moulted regularly. The coelom is small; most of the body cavity is a blood-filled haemocoel.

The digestive system is complete; appendages are modified into mouth parts adapted to different foods. The circulatory system is open, made of a dorsal contractile heart, arteries and the haemocoel. The excretory organ is the Malpighian tubule; coxal, antennal or maxillary glands also occur. Sexes are separate, fertilisation is usually internal, and in most cases there is metamorphosis.

Examples (Figure 1.24): Limulus polyphemus (king crab), Carcinus maenas (crab), Musca domestica (house fly), Culex pipiens (Culex mosquito), Pieris brassicae (butterfly), Periplaneta americana (cockroach), Scolopendra gigantea (centipede), Lycosa lenta (spider).

4. Echinodermata: spiny skin

Echinodermata comes from the Latin echinatus (spinous), the Greek derma (skin) and ata (to bear). Jacob Klein named the phylum in 1734, and 7,550 living species are known. Besides sea stars it holds brittle stars, sea urchins, sea cucumbers and sea lilies. They are triploblastic, true coelomates of organ-system grade.

The skin of every member is spiny. The spines arise from calcareous endoskeletal plates under the skin: the spines are the exoskeleton, the plates the endoskeleton. Unable to osmoregulate, echinoderms are rare in estuaries. All are marine, and there are no parasitic echinoderms. Adults are pentaradially symmetrical and unsegmented, but the larva is bilaterally symmetrical.

The body carries spines and pedicellariae as exoskeleton and is arranged into oral and aboral surfaces; the oral surface has five ambulacral grooves. A water vascular system derived from the coelom is present: its tube feet are the locomotor organs, and the system also helps in respiration and food gathering. There is no blood circulatory system, the haemal and perihaemal systems do its work, and there is no excretory system. Breathing is by dermal gills, tube feet or respiratory trees. They are unisexual, fertilisation is external, and the life cycle has a free-swimming larva.

Examples (Figure 1.26): Antedon bifida (feather star), Ophiothrix fragilis (brittle star), Astropecten euryacanthus (star fish), Crossaster papposus (sun star), Cucumaria planci (sea cucumber), Echinus esculentus (sea urchin), Gorgonocephalus arcticus (basket star), Clypeaster rosaceus (sea biscuit).

Before the side-by-side comparison, open the four body plans: foot, ring, joint, spine. Tap a part.

5. Four phyla side by side

The sharpest differences between the four phyla fall in circulation, movement and body covering. First fix the four kinds of circulation in one line.

These four true-coelom phyla split apart on the last rungs of the key. Pick an animal and walk down.

Now work from the feature to the phylum. Pick up a chip and drop it in the right phylum's box.

Exams give an animal's name and ask for its phylum. Place twelve animals in their phyla.

Last, the story behind each name. The meaning of the word and the year of the namer both come up as short direct questions.

6. Check yourself

Three questions in the exam's style. Start with the first.

The second asks you to match a feature to its phylum.

Take the last one slowly.

The whole note on seven cards.

Source: Biology 2nd Paper (Zoology), Classes 11-12 (NCTB), Chapter 1 “Animal Diversity and Classification”, the “Nonchordata: Mollusca to Echinodermata” part.