In the phylum Arthropoda, there are several classes, two of which are Arachnida and Crustacea. Arachnids and crustaceans are both similar and different in several ways.

First, the classes are similar. Arachnids and crustaceans are both arthropods. They have exoskeletons and jointed legs. They also eat smaller animals.

The classes are also different. Arachnids number more than 60,000 species while there are about 30,000 different crustaceans. Arachnids' bodies have two distinct sections, the opisthosoma and the prosoma. Crustaceans' bodies have three sections: the head, the thorax, and the abdomen. Arachnids are usually less than five centimeters, but crustaceans are larger. Arachnids live mostly on land. They are believed to be the first animals to invade the land from the sea. Crustaceans, however, are mostly aquatic animals. Arachnids use their legs to walk, while crustaceans use theirs to swim, crawl, manipulate food, and perform other tasks. Arachnids are usually nocturnal predators, while crustaceans range from being carnivorous to herbivorous and eat by many means, from filter feeding to scavenging.

Arachnida and Crustacea are two arthropod classes that are both similar and different in several ways, from size to habitat to feeding habits.

First published in Arthropod Blob, 1999.