Morphology and Functional Anatomy

Abstract

Tardigrades are microscopic aquatic bilaterians that despite their size have a complex morphology and anatomy. These minute animals need a surround- ing film of water to be active, and tardigrades residing in terrestrial environments live in moist microhabitats. The latter species are referred to as semiterrestrial, and they endure rapid dehydration by losing water over highly permeable integuments while forming a quiescent tun. Species of the class Eutardigrada are predominantly semiterrestrial and limnic with a relatively uniform morphology and anatomy. Heterotardigrades (class Heterotardigrada) comprising echiniscoideans and arthrotardigrades are much more diverse with highly variable external morpho- logies, ranging from clearly segmented species, over species with extremely large external sense organs and cuticular wings to species with a more “bear”-like eutardigrade body outline. In this chapter, we review the current understanding of tardigrade morphology and functional anatomy and put focus on tardigrade diver- sity emphasizing the often neglected marine arthrotardigrades. We give an intro- duction to the structure and function of major tardigrade organ systems, including the integument, body cavity, and digestive, muscular, nervous, and reproductive systems, and we highlight the diversity of tardigrade sensory appendages and overall external morphology. N.