World Facts

Characteristics of Anthropods

Anthropods exhibit many characteristics that distinguish them from other species. These characteristics include an open circulatory system and jointed appendanges.

Anthropods consist of about 85% of the animals in the world, making them the largest animal categorization. There are many species of Anthropods, including scorpions, spiders, barnacles, insects, centipedes, krill, mites, crayfish, ticks, shrimp, lobsters, and crabs. These species can be categorized into land species and aquatic species. Phylum anthropoda exhibit many characteristics that distinguish them from other species. These characteristics include an open circulatory system, jointed appendages, segmented bodies, excellent senses, and an exoskeleton. Here are the characteristics that define anthropods. 

Open Circulatory System

An open circulatory system is whereby blood is pumped into a haemocoel, and the blood diffuses back to the circulatory system that is between the cells. The heart pumps blood into the body cavities. Blood surrounds tissues in the cavities. This process makes the flow of blood sluggish. Anthropods have a fluid in them that is known as haemolymph. This fluid is a mixture of interstitial fluid and blood. It is this fluid that nourishes organs with oxygen and nutrients in the haemocoel. The muscular movements that these animals exhibit facilitate the movement of haemolymph. However, the diverting flow, which is from one area to another, is limited. Blood is drawn back towards the heart when the heart relaxes through pores that are open-ended. The open circulatory system is suitable for anthropods since it requires very low energy. Anthropods are suited for this type of circulatory system since they have small bodies and a slower metabolism.


Anthropods are considered to be invertebrates since they do not have a backbone or a bony skeleton. These invertebrates rely on their environment to have a balance in their body heat. Despite lacking a bony skeleton, anthropods have some skeleton that is made up from different kinds of materials to make them stronger. Some have their bodies covered with hard materials that are crystalline while others consist of chitin.

Jointed Appendages

An appendage is an attached outgrowth that is from the body of an organism. Therefore, a jointed appendage means that this growth has joints in it. Tetrapod limbs are jointed appendages. Jointed appendages are often used to refer to the segmented appendages of anthropods such as spiders, crabs, and insects. For instance, when we observe the leg of an insect, we see three parts which are the tarsus, tibia, and femur. There is a joint between each of these parts that allows for articulation of the appendage. There exists a set of joints, the trochanter, and the coxa, between the body and the femur. Their antenna is articulated with joints between the body and between the segments. However, some anthropods display unique appendages. For example, crustaceans have branched appendages. However, the branched appendages are still segmented and jointed.

Segmented Bodies

Bodies of anthropods begin to be segmented while they are still embryos. Their bodies are built from modules that are repeated. Each segment of their body has a pair of appendages whose function is similar to those of limbs. The segments are grouped into tagmata. In the tagmata, their limbs and the segments are specialized in different ways. The acron and the teslon are body elements of anthropods that are not part of the segments. The eyes of the anthropods are mounted on the acron. The lower branch of the segmented body is used for locomotion while the upper part is used as a gill. However, some segments have been modified to perform different activities. For example, some have been modified to claws for grasping.

Very Good Senses

Senses are helpful to all animals as they enable them to find members of their species, food and also enable them to avoid predators. Despite being quite small, anthropods have a chemical sense of smell and taste, vision, hearing, sense of cold and heat, and sense of touch. Anthropods have various type of hair that associates them with the sense of touch. When the hair comes into contact with anything, it stimulates its sensory cell that sends signals to its central nervous system. The sense of hearing is associated with hairs that are triggered by sound waves. However, some anthropods like crickets have true tympanic organs. The tympanic organ resembles the ear invertebrates which are made to vibrate when they encounter sound waves. These species have lateral compound eyes that are made up of numerous eyelets. Anthropods have very specialized sensory hairs especially in their mouth which facilitate the sense of taste and smell.


Anthropods have a tough exoskeleton which is the outer shell. The exoskeleton is made up of chitin. Chitin is a material that is closely related to cellulose but very strong. The exciting bit about the exoskeleton is that it does not grow as the anthropod gets bigger like in those that have inside the skeleton. Their adaptation to get through this is that they molt then shed off the skeleton after they have grown a new one which is more substantial. Anthropods add calcium carbonate to their exoskeleton that has chitin to make it stronger. Crabs and lobsters are examples of anthropods that add calcium carbonate to their exoskeleton. Addition of calcium carbonate to an exoskeleton is an adaptive feature which helps these animals a little bit less vulnerable to the bites of their predators. The exoskeleton provides a very protective cover for muscles and the soft internal organs. These creatures become less susceptible to organ and muscle injury. The innermost layer of the exoskeleton is quite waxy and it protects the anthropod against dehydration.

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