Ploimoidaceae; subconical; corona bilobed; retractile foot absent or ciliated; motile appendages present in two families.
The claws are large, strongly compressed, sharp, and exhibit the retractile condition in the highest degree, being drawn backwards and upwards into a sheath by the action of an elastic ligament so long as the foot is in a state of repose, but exerted by muscular action when the animal strikes its prey.
Aeluronyx of Madagascar and Seychelles has cat-like retractile claws.
The mouth opens at the tip of the retractile proboscis; it leads into a short thin-walled tube which opens into an oval muscular gizzard lined with a thick cuticle; at the posterior end of this are some minute glands and then follows a large stomach slightly sacculated in each segment, this tapers through the rectum to the terminal anus.
The antennae are short tubular extensions of the body wall, sometimes retractile with a depressed tip from which protrudes a tuft of fine stiff bristles.
A, Notholca longispina, lorica only; b, Anuraea aculeata, like the former, a floating pelagic type (plankton proper); c, Synchaeta stylata; corona with accessory antennae and sensory styles; auricles for swimming - an actively swimming pelagic type (nekton); d, Pterodina patina, with bdelloid corona and retractile foot with terminal ciliated cup; e, Distyla gissensis partly extended; f, Rattulus tigris.
The tongue is generally beset with more or less scaly or velvety papillae and has always a well-marked posterior margin, while the anterior portion may or may not be more or less retractile into the posterior part.
The anterior one-quarter or one-third of the body is capable of being retracted into the remainder, as the tip of a glove-finger may be pushed into the rest, and this retractile part is termed the introvert.
They may be defined as aquatic animals, forming colonies by budding; with ciliated retractile tentacles and a U-shaped alimentary canal.