Unlike humans and other vertebrates that have complex immune systems, these invertebrates don't. Further, when a pathogen breaches the skin and enters the body, it meets the first line of (systemic ) defense inside the body, namely white blood cells called neutrophils. Neutrophils are always in circulation in the blood and when they meet any foreign body, they attack it by engulfing it (phagocytosis) and kill it.
excretion is wrong guys, i think the correct answer would be lysozyme. have a good day.
Excretion is the only effective way invertebrates fight disease.
Reason: Invertebrates only have the capacity to excrete and cleanse their body of harmful materials and germs.
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a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.