The head is seen in front resting on the foot and carrying a median non-retractile snout or rostrum, and a pair of cephalic tentacles at the base of each of which is an eye.
NEMERTINA, or Nemerteans (Nemertea), a subdivision of worms,' characterized by the ciliation of the skin, the presence of a retractile proboscis, the simple arrangement of the generative apparatus, and in certain cases by a peculiar pelagic larval stage to which the name " pilidium " has been given.
In one genus (Polypocephalus) the place of a rostellum is taken by a crown of retractile tentacles.
Another form, Rhopalophorus, has two cephalic tentacles that are retractile and covered with hooks.
They may be defined as aquatic animals, forming colonies by budding; with ciliated retractile tentacles and a U-shaped alimentary canal.
Tentacles retractile into an introvert ("tentacle-sheath").
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.
Bears are five-toed, and provided with formidable claws, which are not retractile, and thus better fitted for digging and climbing than for tearing.
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.
Aeluronyx of Madagascar and Seychelles has cat-like retractile claws.
- Pleurodont; solid teeth; anterior part of tongue slightly emarginate and retractile, and covered with flat papillae; no osteoderms. Mexico.
- Pleurodont; teeth solid, sometimes (Ophiosaurus) grooved; anterior part of tongue emarginate and retractile into the posterior portion; osteoderms on the body, and especially on the head where they are roofing over the temporal fossa; entirely zoophagous and ovo-viviparous.
Often the head is retractile, and a constriction of flexible cuticle distal to it is termed a neck: in Philodinaceae there are a series of thin flexible rings which permit both distal and proximal ends to be telescoped into the middle; and in Taphrocampa, regular constrictions of the whole bodywall give an appearance of metemeric segmentation to the body.
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.
Ploimoidaceae; subconical; corona bilobed; retractile foot absent or ciliated; motile appendages present in two families.
Cyrenellidae.-Two elongated, united, non-retractile siphons; freshwater.
- Shell sub-trigonal, inequivalve; pallial sinus shallow; siphons short, united, completely retractile; foot large, pointed, often byssiferous.
- Mantle with a fourth aperture; siphons long, quite separate, completely retractile and invertible.
- Siphons separate, naked, completely retractile but not invertible.
- Mantle with fourth aperture; siphons very long, completely united, naked, incompletely retractile; foot small, with posterior appendage.
There is also in some genera a median retractile sensory papilla on the dorsal posterior surface above the rectum, not covered by the cuticle.
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.
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.
The forward-directed exopods either act as valves or form a tube (rarely two tubes), protensile and retractile, for regulating egress of water from the branchial regions.
"The powerful retractile talons of the falcon and the cat tribes have not been produced or increased by the volition of those animals; but among the different varieties which occurred in the earlier and less highly organized forms of these groups, those always survived longest which had the greatest facilities for seizing their prey.
From all other large Carnivora except the African hunting-dog, hyenas are distinguished by having only four toes on each foot, and are further characterized by the length of the fore-legs as compared with the hind pair, the non-retractile claws, and the enormous strength of the jaws and teeth, which enables them to break the hardest bones and to retain what they have seized with unrelaxing grip.