Gastrovascular Cavities vs. Complete Digestive Tracts

With the evolution of multicellularity came a corresponding evolution of cellular specialization, resulting in a division of labor among cells. The cnidarians (formerly called coelenterates) provide a comparatively simple example of this phenomenon. These radially symmetrical animals have a saclike body composed of two principal layers of cells, with a jellylike layer between them. Within the central cavity of this saclike body, extracellular digestion takes place. This cavity has only one opening to the outside, which is surrounded by mobile tentacles that serve to capture prey. A digestive cavity of this sort, with a single opening that functions as both mouth and anus, is called a gastrovascular cavity. Gastrovascular cavities, as the name suggests, function in both digestion and the transport of nutrients to all parts of the body. Organisms belonging to two major phyla, the Cnidaria and the Platyhelminthes, characteristically possess gastrovasular cavities.

Once the food is inside the gastrovascular cavity, digestive enzymes are secreted into the cavity, and extracellular digestion begins. This extracellular digestion, largely limited to proteins in cnidarians, does not break down these substances completely to their constituent amino acids. As soon as the food has been reduced to small fragments, cells lining the gastrovascular cavity engulf them by phagocytosis, and digestion is completed intracellularly in food vacuoles. Indigestible remains of the food are expelled from the gastrovascular cavity via the mouth. The evolution of the additional process of extracellular digestion provides cnidarians with an adaptive advantage: intracellular digestion severely limits the size of the food the organism can handle. Extracellular digestion enables it to utilize much larger pieces of food; even whole multicellular animals become possible prey. Extracellular digestion is the rule rather than the exception in multicellular animals.

Unlike the radially symmetrical cnidarians, the flatworms are bilaterally symmetrical; they have distinct anterior (front) and posterior (rear) ends, and also distinct dorsal (upper) and ventral (lower) surfaces. Their bodies are composed of three well-formed tissue layers. Many flatworms are parasitic, but some are free-living, such as the planaria. The intestine is much more highly branched than that of the cnidarians. The extensive branching greatly increases the total absorptive surface of the cavity and serves to transport food to all parts of the body. Remember that as organisms increase in size, and particularly as their volume increases, the problem of sufficient absorptive surface becomes more acute. Many organisms have evolved greatly subdivided absorptive surfaces for food, thereby compacting much total surface area into relatively little space. The root hairs of plants were one example, and the gastrovascular cavity of the planaria is another. Observe the examples of gastrovascular cavities in the next few stations.

Animals more complex than cnidarians and flatworms have a complete digestive tract—one with two openings, a mouth and an anus. In these organisms incoming food can move in one direction from mouth to anus through a tubular system, which can be divided into a series of distinct sections or chambers, each specialized for a different function. As the food passes along this assembly line, it is acted upon in a different way in each section. The sections may be variously specialized for mechanical breakup of bulk food, temporary storage, enzymatic digestion, absorption of the products of digestion, reabsorption of water, storage of wastes, and so on. The overall result is a much more efficient digestive system, as well as a potential for special evolutionary modifications fitting different animals for different methods of obtaining nutrients. The evolution of a complete digestive tract was a major evolutionary event in the evolution of life.

Nutrient Procurement in Cnidarians

Members of the phylum Cnidaria are a diverse assemblage of aquatic animals distinguished by radial symmetry, the presence of a gastrovascular cavity, and possession of tentacles upon which are located specialized cells called cnidoblasts, each of which produces and contains a stinging body called a nematocyst. The bodies of cnidarians (formerly called coelenterates) consist of an outer epidermis (ectoderm) and an inner gastrodermis (endoderm), with a third layer, the mesoglea (mesoderm), usually occurring between the two. The cnidarians are nearly all marine but one member of this phylum, the hydra, is common in fresh waters. The cnidarians are strictly carnivorous.

Cellular Structure: Observe a prepared slide of a longitudinal section of hydra and examine with high power. Notice the outer thin epidermis layer, an inner thicker, clear gastrodermis layer, and a dark line, the mesoglea, between the two layers. In hydra, the mesoglea is a sheet of gelatinous material with a few scattered cells located between the epidermis and gastrodermis. Note also the central gastrovascular cavity that extends out into the tentacles (see Fig. 1).

The epidermis is composed mainly of medium-sized cells, known as epithelial-muscle cells, whose boundaries are frequently indistinct. The bases of these cells have contractile fibers, but they are not true muscle fibers. Nematocysts are found within many of the epidermal cells.

The gastrodermis is made up chiefly of the nutritive-muscle cells -- large, elongated, vacuolated cells that are generally flagellated. The cells engulf small food particles by phagocytosis, and digestion takes place intracellularly in food vacuoles. Situated between the free ends of the nutritive muscle cells are small triangular cells oriented with their tapered ends pointing toward the mesoglea. These are the enzymatic-gland cells which produce digestive enzymes for extracellular digestion (see Fig. 2).