The pycnogonids, or sea spiders, are small, benthic, marine arthropods, with a superficial resemblance to the true spiders, to which they are probably only distantly related. The body is segmented and covered with an exoskeleton. All pycnogonids are carnivores and use a muscular pharynx to suck soft food into the gut. The mouth is borne at the end of a large proboscis, an unusual feature for an arthropod.
Digestion is intracellular.
Most feed on sponges, cnidarians, or bryozoans from which they suck the fluids. There are no excretory organs, respiratory organs, or coelom. The nervous system is typically arthropodan with dorsal brain, circumenteric connectives, and a double, ventral, longitudinal nerve cord with segmental ganglia. Like that of the chelicerates, the brain is two-part and lacks the deutocerebrum. There is a dorsal tubular heart which pumps blood to the hemocoel.