The size or even presence of these structures depends upon the feeding habits of the organism. In general, parasitic forms have greatly reduced digestive systems, while herbivorous forms have large and elaborate digestive systems. The function is to allow the exchange of gases between the body and the environment. This occurs in different ways depending upon habitat and size of the organism. Certain cells also participate in immunological response to foreign substances or pathogens.
Capillaries are the site of gas exchange.
Blood is pumped by a heart that can have from 2 to 4 chambers. In amniotes, individuals are one sex or the other and often determined chromosomally. The function is that of integration of information and response throughout the body and the sensing of events in the external environment. In addition, the autonomic part of the nervous system is divided into sympathetic and parasympathetic . The chemical messengers are hormones and they are released by endocrine glands.