Sponges, cnidarians and other early-diverging animal lineages

A sponge's canals connect specialised cells

In a selected suspension-feeding sponge, beating choanocyte cilia or flagella drive water through channels and feeding chambers. Choanocytes are collar-bearing cells that capture small food particles and take them into the cell. Water then exits through larger openings. The path past feeding cells is not a gut in which a large meal is digested.

Other sponge cells contribute to covering, support, nutrient distribution and renewal. The mesohyl is a region of extracellular material and cells between surfaces. Skeletons can include mineral spicules or organic fibres, with substantial differences among lineages. Body contractions and cellular signalling help coordinate activity despite the absence of a conventional nervous system.

Descriptions of sponges as having 'no true tissues' depend partly on how tissue is defined. Modern work identifies differentiated cell populations and organised tissue regions. The useful contrast is the actual arrangement and coordination of cells, not an absolute claim that a sponge is an unorganised pile. Most sponges filter feed, but carnivorous lineages show why this feeding route is a selected case rather than a universal animal plan.

Cnidarians combine surfaces, a gut and specialised capture cells

Cnidarians include sea anemones, corals and jellyfish. Their characteristic cnidocytes contain specialised capsules used in capture or defence; a nematocyst is one type of capsule, not the whole cell. Many cnidarians have tentacles around a mouth and a digestive space where food processing begins before nutrients enter cells.

A polyp commonly presents the mouth and tentacles away from its attached base. A medusa commonly has a bell-shaped body with the mouth directed beneath it. In the selected body comparison, an outer tissue layer and a gut-facing layer enclose a jelly-like mesoglea. Both spaces open to the same surrounding water through a mouth, but their positions differ.

Some cnidarian life cycles include both polyp and medusa forms, while others do not. Sea anemones lack a medusa stage. Neither the polyp nor the medusa is an animal equivalent of a plant's haploid gametophyte. In life cycles that contain both forms, they are diploid body stages; gametes are produced through meiosis.

A similar outline can conceal a different history

Ctenophores are comb jellies, a lineage distinct from cnidarians. Rows of cilia on comb plates provide conspicuous locomotor structures in many forms. Their nervous systems, feeding structures and development require comparison in their own right. A transparent, jelly-like body is not sufficient evidence to unite all such animals into one exclusive clade.

Placozoans are another living animal lineage with flattened bodies and differentiated cells. Their small bodies do not represent a stage through which every other animal must pass. Likewise, a sponge living today is a descendant of ancient animal populations, not a frozen version of the ancestor shared with a jellyfish.

Early-diverging is a statement about a branch relative to another branch on a rooted tree. All living lineages have continued to change since their common ancestors. A body with fewer familiar organs can be highly effective in its present environment, while its evolutionary history can include retained, modified and lost features.

Worked example

Check your understanding

1. The sponge chamber is full of water. Why is it misleading to label it a digestive gut?

   Show reasoning

   In the selected sponge, particles are captured and digested within cells along the water route. The canal or chamber carries water past those cells; it is not the digestive cavity of a cnidarian.

2. If only 12 of 20 assigned particles leave the sponge, why does the water outlet still carry 6 volume units?

   Show reasoning

   The ledgers track different quantities. Eight particles are captured, while the constructed water account assumes no storage change and conserves the 6 entering volume units. Particle capture does not equal loss of the same number of volume units.

3. Does a polyp-to-medusa life cycle prove alternation between haploid and diploid multicellular bodies?

   Show reasoning

   No. In the selected cnidarian life cycles both polyp and medusa are diploid forms. Body shape and attachment can change without a ploidy change; gametes are the haploid cells.

What to carry forward

Trace the material and cell boundary before comparing bodies; living early-diverging animals retain their own evolving histories.