Class Hydrozoa: the freshwater hydra
Coelenterates are simple many-celled animals whose body has two cell layers; the body cavity works as a gut. The hydra lives in clean still freshwater, is 5–7 mm long, attaches by its foot, and at the opposite end has a mouth ringed by tentacles. The body can be divided into equal parts around an imaginary axis – this is radial symmetry. The outer layer (ectoderm) holds skin-muscle, stinging, nerve and interstitial cells; the thread in a stinging cell’s capsule pierces and paralyses prey. Nerve cells form a nerve net, so the hydra contracts its body in response to a stimulus – a reflex. Flagellate and gland cells of the inner layer (endoderm) produce digestive juice and digest food. Thanks to interstitial cells the hydra can regrow from body pieces (regeneration). It reproduces by budding (asexual) in summer and by eggs and sperm (sexual) in autumn; the overwintering zygote gives a young hydra in spring.
Make a paper model of a hydra (a two-layer “bag” with tentacles) and draw its axis of radial symmetry. Safety: only the teacher or another adult collects the water for viewing hydra; use scissors with an adult’s help; never go to a pond alone.