Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Metazoa
Coelenterates
Class Hydrozoa - Subclass Hydroida

A typical representative is the freshwater hydra (Hydra oligactis). Hydras are among the simplest solitary freshwater polyps, frequently found in lakes and ponds. The hydra has an elongated, sac-like body (Fig. 12), with its free end bearing a Mouth surrounded by a ring of 6–12 tentacles.

The expanded part of the hydra's body is called the central gastric region. It narrows into a slender stalk terminating in a basal disc, which the hydra uses to attach itself to the stems and leaves of aquatic plants.

The ectoderm and endoderm of the hydra consist of A large number of Cells of various types. Epitheliomuscular cells make up the bulk of them. The ectoderm contains nematocysts ( stinging cells), which are predominantly located on the tentacles. The hydra uses them for defense, as well as to capture and paralyze prey. Sensory and Nerve Cells, located in the ectoderm, ensure the perception of stimuli and transmit them to other cells.

Glandular cells that secrete adhesive substances are concentrated mainly in the ectoderm of the basal disc and tentacles. Between all these cells lie small, undifferentiated interstitial cells that can transform into any other Cell type when necessary. For example, if the Organism's integrity is compromised, its restoration (regeneration) occurs through these cells.

In the hydra's endoderm, alongside epitheliomuscular, nerve, and sensory cells, there are numerous glandular cells that secrete digestive Enzymes. Many epitheliomuscular cells can form flagella and pseudopodia, which are used to capture food particles for intracellular Digestion.

Hydras are predators. They feed on small crustaceans, tiny worms, and the like. After capturing prey with its tentacles, the hydra pushes it through the mouth into the gastrovascular cavity. Under the action of secretions from the endodermal glandular cells, the food is softened and turned into a mushy consistency. This is how extracellular (coelenteric) digestion takes place. Next, small food particles are engulfed by cells that form pseudopodia, and the digestive process is completed intracellularly.

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Fig. 12 Stalked hydra:

A — longitudinal section; B — cross section; 1 — oral cone; 2 — mouth opening; 3 — male reproductive Gonads; 4 — coelenteron (gastric cavity); 5 — ovum (egg cell); 6 — basal disc; 7 — bud; 8 — ectoderm; 9 — glandular cells; 10 — interstitial cells; 11 — epitheliomuscular cells; 12 — mesoglea (supporting lamella); 13 — endoderm.

Fig. 13 Part of a marine hydroid colony (longitudinal section):

1 — hydranth in extended state; 2 — contracted hydranth; 3 — protective sheath (perisarc / theca); 4 — hydrotheca (cup); 5 — body of the hydranth (ectoderm, endoderm, and gastric cavity extending into the stem are visible); 6 — bud; 7 — blastostyle with developing medusae on it; 8 — gonotheca.

Hydras reproduce both asexually and sexually. In summer, under optimal conditions, budding takes place. A bud forms as an outward protrusion of the body walls. Subsequently, a mouth opening breaks through at its distal end and tentacles are formed, followed by The Development of the basal disc, after which the young hydra detaches from the maternal organism. Sexual reproduction in the hydra usually occurs in autumn, with the onset of unfavorable conditions. Gonads form as small bumps in the ectoderm. In hermaphroditic forms, they develop in different locations: Testes develop closer to the oral pole, while Ovaries form closer to the basal disc. The maturation of male and female Gametes occurs at different times, which ensures cross-Fertilization. The fertilized egg (zygote) develops tough outer coats, sinks to the bottom, and overwinters there. The following spring, a new generation of hydras hatches from it.

Marine hydroids also belong to the Class Hydrozoa. Only very few of them lead a solitary lifestyle like the hydra. Typically, they form tree-like or shrub-like colonies. Individual members of the colony, called hydranths, sit on their branches (Fig. 13). In Structure, they correspond to the hydra—they form through budding, but do not detach from the main stem.

The digestive cavity of the hydrans continues into a canal running through the entire stem and Branches of the colony. Thus, food captured by any hydranth can be distributed throughout the entire colony.

Blastostyles also form on the stems of the colony via budding, upon which sexual individuals—hydromedusae—develop. They have an umbrella-like shape up to 3 cm in diameter. The walls of the medusa's body consist of the same layers and cells as those of the hydra. However, the Digestive System is somewhat more complex due to The formation of radial canals through which food is delivered to all PARTS OF THE body.

Due to the free-swimming lifestyle of hydromedusae, The structure of their Nervous system and Sense Organs becomes more complex: a nerve ring, ocelli (simple eyes), and balancing organs—statocysts—appear.

Germ Cells form within the ectoderm of the hydromedusae; upon maturation, they are released into the Water through a rupture in the outer body wall, where fertilization takes place. A planula larva emerges from the zygote. After swimming for some time, it settles on the bottom and develops into a polyp.