Your Backbone Started Out as a Simple Rod in the Embryo

A sponge, an earthworm, a butterfly and a tiger look so different that grouping them seems hopeless. But beneath the surface, all animals can be compared by a few basic features of body design.

Those features — how cells are organised, how the body is shaped, how many germ layers form and whether there is a body cavity — become the keys that sort every animal phylum.

This part covers levels of organisation, symmetry, germ layers and coelom, and segmentation and the notochord.

Animals range from the cellular level, where cells work loosely together, through the tissue level, where similar cells form tissues, and the organ level, where tissues form organs, to the organ-system level, where organs work together in systems.

- Cellular level — cells in loose aggregates with some division of labour: sponges (Porifera)
- Tissue level — cells with the same function arranged into tissues: coelenterates such as Hydra, and ctenophores
- Organ level — tissues grouped into organs: flatworms (Platyhelminthes)
- Organ-system level — organs linked into systems: roundworms, annelids, arthropods, molluscs, echinoderms and chordates

Patterns within organ systems.

- Digestive system: incomplete, with one opening serving as mouth and anus, in flatworms; complete, with separate mouth and anus, in most higher groups
- Circulatory system: open, where blood flows into body spaces, in arthropods and molluscs; closed, where blood stays in vessels, in annelids and chordates

Worked example — placing animals. A sponge is at the cellular level, Hydra at the tissue level, a tapeworm at the organ level and a frog at the organ-system level.

An everyday example. A school is organised in layers — students form sections, sections form departments, and departments together run the whole school.

The substance. Level of organisation is about how cells are arranged, not how big the animal is.

Asymmetrical animals cannot be cut into equal halves by any plane through the centre; radially symmetrical animals give identical halves along any plane through their central axis; and bilaterally symmetrical animals give matching left and right halves along only one plane.

- Asymmetrical — most sponges
- Radial symmetry — coelenterates, ctenophores and adult echinoderms such as starfish
- Bilateral symmetry — flatworms, roundworms, annelids, arthropods, molluscs and chordates

Symmetry and mode of life.

- Radial animals are usually fixed or slow-moving; they meet food and danger from every direction, so a body arranged all round a centre suits them
- Bilateral animals usually move actively in one direction, with a front end that meets the world first; sense organs gather there, forming a head

Worked example — decide the symmetry. A jellyfish can be cut into equal halves along many lines through its centre — radial. A cockroach has only one line that gives mirror-image halves — bilateral.

An everyday example. A round slice of cucumber can be cut into two equal halves in many directions, like a radially symmetrical animal, while your face splits into matching halves only down the middle.

The substance. Echinoderm larvae are bilaterally symmetrical, even though the adults are radial.

Diploblastic animals develop from two germ layers, ectoderm and endoderm, with an undifferentiated mesoglea between them, while triploblastic animals add a middle layer, mesoderm; a true coelom is a body cavity lined by mesoderm, a pseudocoelom is a cavity whose mesoderm is scattered as pouches, and acoelomates have no body cavity.

Germ layers:

- Diploblastic — ectoderm and endoderm with mesoglea between: coelenterates
- Triploblastic — ectoderm, mesoderm and endoderm: flatworms onwards

Body cavity:

- Coelomates — cavity fully lined by mesoderm: annelids, molluscs, arthropods, echinoderms, hemichordates, chordates
- Pseudocoelomates — cavity present, but mesoderm is scattered in pouches between ectoderm and endoderm: roundworms (Aschelminthes)
- Acoelomates — no body cavity: flatworms (Platyhelminthes)

Worked example — classify. Hydra — diploblastic, no coelom. A tapeworm — triploblastic, acoelomate. Ascaris — triploblastic, pseudocoelomate. An earthworm — triploblastic, coelomate.

An everyday example. A vacuum flask has two walls with a space between them — much as a true coelom is a cavity lined on both sides by mesoderm.

The substance. A coelom lets internal organs move and grow independently of the body wall, which helps larger, more active animals.

Metameric segmentation divides the body, inside and out, into repeating segments, as in the earthworm; the notochord is a rod-like structure formed from mesoderm along the back during embryonic development, and its presence separates chordates from non-chordates.

Segmentation. In the earthworm, rings on the outside match repeated sets of some organs inside — a true metameric body plan.

Chordates versus non-chordates:

- Notochord: present versus absent
- Nerve cord: dorsal, hollow and single versus ventral, solid and double
- Pharyngeal gill slits: present versus absent
- Heart: on the ventral side versus on the dorsal side, if present
- Post-anal tail: present versus absent

Worked example — sort the animals. A fish and a frog have a notochord in the embryo and a dorsal hollow nerve cord — chordates. A cockroach has a ventral, solid, double nerve cord and no notochord — non-chordate.

An everyday example. The rings along an earthworm dug from garden soil are visible signs of its segmented body.

The substance. In vertebrates the notochord is replaced by the backbone as the embryo develops, so adults do not show it.