A phylum of microscopic ecdysozoan invertebrates, closely related to ARTHROPODS. Members exhibit anabiosis and cryptobiosis, dormant states where metabolic activity is reduced or absent, thus making them tolerant to extreme environmental conditions. They are distributed worldwide and most are semi-aquatic.

Measurement of the OSMOLARITY of solutions or BODY FLUIDS.

Members of the phylum Arthropoda, composed of organisms having a hard, jointed exoskeleton and paired jointed legs. It includes the class INSECTS and the subclass ARACHNIDA, many species of which are important medically as parasites or as vectors of organisms capable of causing disease in man.

Animals that have no spinal column.