Animal Kingdom — Tier 2, Part A

Every animal is made of cells. The question this criterion asks is: how much do those cells cooperate? There are four possible answers, and NCERT arranges the phyla along them.

At the cellular level, cells are simply arranged in loose groups. Each cell largely feeds itself and there is very little division of labour. This is Porifera, the sponges — and only Porifera.

At the tissue level, cells performing the same function are grouped together into a tissue. There is real teamwork now, but no organs yet. This is Coelenterata and Ctenophora.

At the organ level, tissues of different kinds are assembled into organs, each with a specific job. This is Platyhelminthes — and only Platyhelminthes.

At the organ-system level, organs are linked into systems that work together. This is Aschelminthes and everything after it. Once a phylum reaches this level, NCERT grades it further on two things: whether the digestive tract is incomplete (one opening) or complete (mouth and anus), and whether the circulatory system is open (blood sloshing in cavities) or closed (blood confined to vessels).

Symmetry asks a simple question: can you cut this animal into two matching halves, and if so, in how many ways?

If no cut works, the animal is asymmetrical. Sponges are the example — a sponge grows into whatever shape the rock allows.

If many cuts work, provided each passes through the central axis, the animal is radially symmetrical. Think of a wheel, or a jellyfish seen from above. This is Coelenterata, Ctenophora and adult Echinodermata.

If exactly one cut works — the one down the midline, giving a left half and a right half — the animal is bilaterally symmetrical. This is Platyhelminthes and everything after it, including us.

There is a reason behind the pattern, and knowing it makes the list stick. Radial symmetry suits an animal that stays put and meets food arriving from any direction, so it needs to be equally ready on all sides. Bilateral symmetry suits an animal that travels head-first: it can concentrate sense organs and a brain at the leading end, which is called cephalisation. Sessile animals go radial; movers go bilateral.

That reasoning also explains the famous exception. An echinoderm larva swims, so it is bilateral. The adult settles on the sea floor, so it becomes radial. The animal changes lifestyle, so it changes symmetry.

Very early in development an animal embryo sorts itself into layers of cells called germ layers. Every organ the adult will ever have is built from one of them. The question this criterion asks is simply: two layers, or three?

Diploblastic animals build two: an outer ectoderm and an inner endoderm. Between them sits a jelly-like material called mesoglea. Mesoglea is the detail that catches students out — it is not a germ layer. It is undifferentiated packing, with no cells organised into tissues. Coelenterata and Ctenophora are the diploblastic phyla.

Triploblastic animals build three: ectoderm, mesoderm and endoderm. Mesoderm is a genuine third layer of cells, and it is the important one. From it come muscle, skeleton, blood and the circulatory system, the kidneys, and the gonads. Platyhelminthes and everything after it are triploblastic.

Now see why this criterion sits where it does in the decision tree. A coelom is defined as a cavity lined by mesoderm. If an animal has no mesoderm, the question 'is the cavity lined?' cannot even be asked. That is why NCERT's table simply records the coelom as absent for Porifera, Coelenterata and Ctenophora, rather than calling them acoelomate.

One more thing worth noticing. Ectoderm always makes the outer covering and the nervous system; endoderm always lines the gut. Those two jobs are the same in both plans. All the extra machinery of a complex animal comes from the third layer. That single sentence explains why triploblastic animals could become large, muscular and organ-rich, and diploblastic ones could not.

A sponge is an animal that gave up moving and built a plumbing system instead. That single idea explains almost everything in this section.

Water enters through many tiny pores called ostia scattered over the body wall, passes into a large central cavity called the spongocoel, and leaves through a single large opening at the top called the osculum. This is the water transport system, also called the canal system. Note the direction: many small holes in, one big hole out.

That one current does four jobs at once: it brings food, it brings oxygen, it carries away waste, and it carries gametes. A sponge does not need a mouth, gills, kidneys or a circulatory system, because the water does all of it.

Driving the current are choanocytes, also called collar cells, which line the spongocoel and the canals. Each has a whip-like flagellum, and together they beat the water through. Food particles are trapped and digested inside individual cells — intracellular digestion, which follows directly from the cellular level of organisation.

For support, a sponge lays down a skeleton of spicules (needles of calcium carbonate or silica) or of spongin fibres, which are protein. The bath sponge is the spongin skeleton of Euspongia, cleaned of its cells.

Reproduction: sponges are hermaphrodite — the same individual makes both eggs and sperm. Fertilisation is internal, and development is indirect, with a free-swimming larva that looks nothing like the adult. The larva is how a sessile animal manages to spread.