Biology 2nd Paper (Zoology) · Classes 11-12 · Chapter 1
Nonchordata: Porifera to Nematoda.
A bath sponge was once a living animal with no tissues at all. A round worm in the gut, on the other hand, has a full digestive tube from mouth to anus. The first four nonchordate phyla (Porifera, Cnidaria, Platyhelminthes and Nematoda) show the ladder between these two ends.
1. Porifera: the pore-bearing sponges
Porifera comes from Latin porus (pore) and fer (to bear), so it means pore-bearer. Robert Grant named the phylum in 1836, and 8,659 living species are known so far.
These are the oldest and simplest animals. The many pores in the body give them the name pore-bearers, and they are commonly called sponges. Most live in the sea; only family Spongilidae lives in fresh water.
The body is made of many cells, but the cells do not form tissues, so these are animals of cellular grade. The pores in the body wall are called ostia. There is no circulatory system; a canal system carries a stream of water instead, and food, oxygen and sperm enter the body with it.
The framework of the body is built of spicules (small calcareous spines) or spongin (an organic fibre). The inner wall has chambers lined by special flagellated cells called choanocytes, and the chambers open into the canal system. The canals join a wide cavity, the spongocoel, which opens to the outside through a large apical opening, the osculum.
An adult sponge is sessile, fixed to an object for good, but its life cycle has a free-moving Amphiblastula or Parenchymula larva. Examples: Scypha gelatinosum (urn sponge), Cliona celata (red sponge), Spongilla lacustris (freshwater sponge) and Euspongia officinalis (bath sponge).
2. Cnidaria: flowers of the sea
Cnidaria comes from Greek knide (nettle) and Latin aria (connected with). Hatschek named the phylum in 1888, and 10,203 living species are known.
They are radially symmetrical and of cell-tissue grade. Most are sessile and some swim freely, like jellyfish. Corals and reef builders belong here, which is why Cnidarians are called flowers of the sea. Coral reefs shelter 25% of marine species, so a coral reef is called the Rain Forest of the sea.
The body wall is diploblastic: epidermis outside, endoderm or gastrodermis inside, and between them a sticky, jelly-like acellular layer, the mesoglea. Cnidocytes hold the nematocysts and are most numerous on the tentacles. The nematocyst is the stinging organelle, used for defence, food capture and cleaning the body.
The only gastrovascular cavity is the coelenteron. A single opening works as both mouth and anus. Food is digested both outside and inside the cells.
They show polymorphism: the polyp is sessile and reproduces asexually, while the medusa is free and reproduces sexually. The larva is the ciliated Planula. Only twenty species live in fresh water; all the rest are marine.
Examples: Aurelia aurita (jellyfish), Porpita porpita (blue button), Gorgonia ventalina (sea fan), Physalia physalis (floating hermit), Cerianthus filiformis (tube anemone), Pennatula sulcata (sea pen) and Tubularia rosa.
3. Platyhelminthes: the flatworms
Platyhelminthes comes from Greek platys (flat) and helminth (worm). Minot named the phylum in 1876, and 29,487 living species are known. These are the simplest and first triploblastic animals, and their bodies show the first tissue-organ grade. Most are endoparasites of humans and other animals.
The body is soft, bilaterally symmetrical and flattened dorso-ventrally like a leaf or ribbon. The skin is a ciliated epidermis or a cuticle. Though triploblastic, they are acoelomate: the spaces between organs are filled with connective tissue called parenchyma (mesenchyme), and there is no internal cavity other than the digestive one.
There are no circulatory or respiratory systems. The excretory system is made of branched excretory tubes and flame cells. Many have external suckers, hooks or both. Most are parasites, and some are free-living. They are hermaphrodite, fertilisation is internal, and the life cycle has many larval stages.
Examples: Schistosoma mansoni (blood fluke), Bipalium kewense (hammerhead flatworm), Fasciola hepatica (sheep liver fluke), Gyrodactylus corti (salmon worm) and Taenia solium (pork tapeworm).
4. Nematoda: the thread worms
Nematoda comes from Greek nematos (thread), eidos (form) and helminth (worm). Gegenbaur named the phylum in 1859, and 25,033 living species are known. These are thread worms or round worms of organ-system grade. Most are parasites, and they are the most numerous of the pseudocoelomate animals.
Parasitic nematodes badly harm humans, cattle and crops, while free-living ones feed on bacteria, yeast, fungi and algae. The body is cylindrical and bilaterally symmetrical, tapering at both ends and broad in the middle. Length runs from microscopic to one metre. The animal is pseudocoelomate and unsegmented.
The body is covered by a thick, acellular cuticle of elastin, and the mouth is usually ringed by lips. The gut is straight and unbranched, running from mouth to anus, so the body looks like a tube within a tube. Respiratory and circulatory systems are absent.
Most nematodes are unisexual and show sexual dimorphism. When the male and female of a species differ in colour, size, shape or structure, this is sexual dimorphism.
Examples: Loa loa (eye worm), Ascaris lumbricoides (round worm), Trichinella spiralis (trichina worm) and Wuchereria bancrofti (filaria worm).
5. The four phyla together
Open the four body plans one at a time. Tap a part to see what it is and what it does.
Four body plans
The drawings are hand-drawn models, not to scale.
Now place the terms in their phyla yourself. Every term comes from one of the paragraphs above.
Whose feature?
Before naming phyla from names, take the key in hand. The first four questions are enough to split these four phyla.
Question by question to the phylum
Exams often give only an animal's name and ask for the phylum. Place ten names in their phyla.
Which phylum?
The body plan gets one step more complex with each phylum. Put them in order from simple to complex.
Simple to complex
A phylum's name tells you what it looks like. Turn the cards to check.
Story of the name
6. Check yourself
The first question is in the exam's style.
Question 1
Which animal is rich in ostia?
The second asks you to spot the phylum from a feature.
Question 2
Which of the following is an animal of phylum Platyhelminthes?
In the last one, read the words closely.
Question 3
What is the difference in colour, size, shape or structure between male and female nematodes called?
The whole note on seven cards.
Seven cards
Source: Biology 2nd Paper (Zoology), Classes 11-12 (NCTB), Chapter 1 “Animal Diversity and Classification”, the “Nonchordata: Porifera to Nematoda” part.