Mechanism of Digestion in sponges:
Sponges don’t have distinct digestive system. They engulf food by the support of water flow system. They show filter feeding behaviour, where food particle filter out of the water passing through them. Only particle smaller than 50 micrometre can enter through ostia. Sponges consume food by phagocytosis with the help of pinacocytes or archaeocytes. Food particles smaller than 0.5 micrometre can catch and consumed by choanocytes.

Mechanism of Digestion in Lower Vertebrate such as Cockroach:
Cockroach searches the food with the help of antennae, maxillary and labial palps. These appendages bear sense organs. With the help of labium and labrum, the pro-legs pick up and bring food to the pre-oral cavity. Mandibles contain teeth which help in mastication of food in pre-oral cavity. The lacinia present in the maxillae also helps in mastication. The food is mixed with saliva in pre-oral cavity. The saliva of cockroach contains amylase, chitinase and cellulose which digest carbohydrate partially in pre-oral cavity. Food from pre-oral cavity is then transported to esophagus and then into the crop. Crop is analogous to stomach of human, which store food for some time and also digestion takes place. Then food reaches to gizzard for crushing into fine particles, which then passes to midgut. Most of the digestion carried out in midgut. Digested food is absorbed by the inner lining of midgut through diffusion. The undigested food passes to hindgut. Water and electrolytes absorb here and undigested liquid food is converted to semisolid faeces, which the passed out through anus in the form of small dry pellets. The complete digestive system of cockroach is shown in Figure 11.3.

MECHANISM OF DIGESTION IN HIGHER VERTEBRATE (SUCH AS HUMAN):
(1) INGESTION: The amount of food that an individual ingest in depend on the intrinsic desire for food (known as hunger) and the type of food that an individual seeks is determined by appetite. For maintaining an adequate supply of nutrition to the body these mechanisms are important. In most of the higher animals there are mainly two mechanism of ingestion: 1) Mastication or chewing and 2) Deglutition or swallowing.