The advantages of the heterotrophic method of nutrition are: the food usually contains all the appropriate chemicals needed by the consumer "feeding" is not restricted to daylight hours.
The disadvantages are:
1) The consumer has to find food by one of the following:
active hunting - requiring well developed systems: a) locomotion and b) sensory and nervous systems. b) passive accumulation - e.g. filter feeding by sedentary animals. c) passive dispersal onto a suitable food source, e.g. fungal and bacterial spores. May have to overcome prey defences (e.g. spines, prickles, noxious chemicals) and therefore might need weapons of attack or immunity to these chemical defences. The food has to be made usable by the consumer by a process of digestion. Material that is indigestible will have to be voided.
Foods are usually dead or alive body parts and generally consist of large, complex organic molecules that need breaking down to smaller chemically simpler forms. small, simple organic and inorganic molecules and ions which just need to be dissolved. other materials which are usually not usable (e.g. lignin, wax, chitin) and which must be discarded.
The amount of physical and/or chemical breakdown necessary depends on the size and composition of the food; as a consequence the nature of the digestive systems will vary somewhat according to the nature of the food. Digestion occurs externally in decomposers (fungi, bacteria) and carnivorous plants. These organisms secrete digestive enzymes onto the food on/or through which they are growing (bacteria, fungi) or which they have trapped (carnivorous plants). The enzymes break down the food and the products are absorbed by the consumer.
In animals digestion is usually internal therefore the process needs to be separated from the body in order to prevent autodigestion. intracellular digestion - e.g. in Protozoa, like Amoeba , digestion occurs within food vacuoles within the cytoplasm. Food is engulfed by the cells, forming a vacuole. Enzymes are secreted into the vacuole, the food item is digested and the products of digestion are absorbed and any undigested material is eliminated when Amoeba flows' away.
extracellular digestion occurs in multicellular animals in a special body compartment, the gastro-intestinal tract or the gut.
The simplest guts have only 1 opening (the mouth) e.g. Hydra, flatworms. Feeding can only be intermittent as it is limited by gut capacity. Food is taken in through the mouth, digested in the inner cavity of the animal then the waste is eliminated back out through the mouth. More complex guts have 2 openings (the mouth and the anus), allowing for more efficient continuous processing of food. All these guts are modifications of a basic plan; a muscular tube whose parts have become specialised for particular functions: buccal cavity: for the reception/apprehension of food, it may be muscular (e.g. worm) or it may have mouth parts for apprehension of food and its physical breakdown (e.g. teeth, mandibles). stomach is for storage, it is also the site of some digestion in some animals. In some animals (e.g. worm, bird) it is divided into 2 parts, an anterior, thin-walled crop for storage and a posterior thick-walled muscular part, the gizzard, for grinding up food. diverticulae (or caecae) are blind-ending tubes which are often secretory. In some animals they have become modified to form the pancreas and liver. Intestine is the main site of digestion and absorption of digested material. In some animals the posterior part may form the rectum for storage...