Copepods

Because of their small size, copepods have no need of any heart or circulatory system (the members of the order Calanoida have a heart but no blood vessels), and most also lack gills, being able to absorb oxygen directly into their bodies. Their excretory system consists of maxillary glands.
Copepods have a variety of sensory capabilities. The most noteworthy are bristle-like setae that act as mechanoreceptors responding to flow that causes bending. An array of such sensors allows detection of patterns of water flow around the body caused by approaching prey or predator, and the copepod can distinguish between the two. The sensors are highly specialized for sensitivity and the nerves are even myelinated for fast conduction.