Hydra – Internal Anatomy

Hydras belong to the phylum Cnidarian, and their body structure is relatively simple. Their body contains a gastrovascular cavity, which serves as the main chamber for digestion. This cavity has only one opening, which functions both for feeding and expelling undigested material. Because of their lack of rigid structures, hydras can significantly expand their body volume, allowing them to digest prey up to twice their own size. The entire digestive process typically lasts between two and three days. Hydras lack specialized respiratory and excretory systems. Instead, excretion occurs directly through the epidermis. Their nervous system is diffuse, meaning it consists of interconnected cells distributed evenly throughout the body. These nerve cells link to photoreceptors and touch-sensitive cells located in the body wall and tentacles. Hydras are also highly regenerative and can replace injured or missing body parts effectively. Some hydra species display a vivid green coloration, which is not due to pigmentation. Rather, the green color comes from green algae living symbiotically within their cells. This relationship is mutually beneficial: the algae find a safe habitat within the hydra, while the hydra benefits from the photosynthetic products produced by the algae.