Nutrient Procurement By Coelenterates
With the evolution of multicellularity came a corresponding evolution of cellular specialization, resulting in a division of labor among cells. The coelenterates provide a comparatively simple example of this phenomenon and they can be easily examined using teaching microscopes. When viewed under a microscope such as teaching microscopes, these radially symmetrical animals have a saclike body composed of two principal layers of cells, with a jellylike layer, called mesolamella (”middle 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. A digestive cavity of this sort, with a single open¬ing that function as both mouth and anus, is called a gastrovascular cavity.
Coelenterates are strictly carnivorous. Embedded in their tentacles are numerous stinging structures called nematocysts. Each nemato¬cyst consists of a slender thread coiled within a capsule, with a tiny hairlike trigger penetrating to the outside, as seen under teaching microscopes. When appropriate prey comes in contact with the trigger, the nematocyst fires, the thread turns inside out, spines on its surface unfold, and it either penetrates the body of the prey or entangles it in sticky loops. The nematocysts also eject poisons, which have a paralyzing action on the prey. The tentacle then grasps the prey, and if it continues to struggle, neighboring tentacles may also become involved. The tentacles draw the prey toward the mouth, which opens wide to receive it.
Once the food is inside the gastrovascular cavity, digestive enzymes are secreted into the cavity, and extracellular digestion begins. When examined using teaching microscopes, this extracellular digestion, largely limited to proteins in coelenterates, 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, as seen under a microscope. Indigestible remains of the food are expelled from the gastrovascular cavity via the mouth.
If phagocytosis and intracellular digestion are going to take place anyway, we can ask what adaptive advantage the evolution of the additional process of extracellular digestion might have. Why should not coelenterates rely exclusively on intracellular digestion, as Pro¬tozoa do? The answer is obvious. 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, as evident when they are examined under a microscope.
NUTRIENT PROCUREMENT BY FLATWORMS
Unlike the radially symmetrical coelenterates, the flatworms, when examined under a microscope, are bila¬terally symmetrical; they have distinct anterior (front) and posterior (rear) ends, and also distinct dorsal (upper) and ventral (lower) sur¬faces. Their bodies are composed of three well-formed tissue layers. Many flatworms are parasitic on other animals, but some are free-liv¬ing, and it is to these we shall turn first, using planaria as an example
The mouth of planaria is located on the ventral surface, near the middle of the animal. It opens into a muscular tubular pharynx, which can be protruded through the mouth directly onto prey. The pharynx leads into a gastrovascular cavity (a cavity with only one opening to the outside). This cavity, though functionally similar to that of the coelenterates when examined under a microscope, branches throughout the animal’s body. The extensive branching greatly increases the total absorptive surface of the cavity and serves to transport food to all parts of the body. We saw with plants that as organisms increase in size, and particularly as their volume increases, the problem of sufficient absorptive surface becomes more acute, as seen when examined under a microscope. Many organisms have evolved greatly subdivided absorptive surfaces, thereby compacting much total surface area into relatively little space. The root hairs of plants were one example, and the branched gastrovascular cavity of planaria is another; we shall encounter many more in this and later chapters.
The members of one class of flatworms, the tapeworms, have be¬come so highly specialized as parasites living in the digestive tracts of other animals that in the course of their evolution they have lost their own digestive systems. They are constantly bathed by the products of the host’s digestion and can absorb them without having to carry out any digestion themselves. Evolutionary adaptation can involve the loss of structures as well as their acquisition.
