[Illustration: FIG. 109.—_Callidina symbiotica._ (After Zelinka.) A, Ventral view, with the disc half expanded, proboscis extended; B, lateral view, proboscis extended; C, ventral view of anterior segments with expanded disc; D, lateral view of same (proboscis retracted). _a_, Antenna; _bl_, bladder (enlargement of rectum); _c_, ciliated cup of the proboscis; _ci_, cingulum; _cl_, cloaca; _cp_, group of pores, the openings of cement glands; _di_, disc; _g_, gizzard; _gm_, germarium (that of the opposite side seen at a higher level); _gr_, ciliated groove; _k_, kidney; _l_, lip; _m_, mouth; _pr_, proboscis; _sp_, spurs of foot; _tr_, large cilia of trochus, showing vertical movements; _vm_, yolk-gland. The body muscles are represented by shaded bands.] The head is very frequently retractile, as a whole, by strong muscles. In Bdelloida the disc proper is retracted when the animal crawls, while the proboscis is exserted (Fig. 109). Ciliated patches occurring outside the region of the disc point to {205}a primitive condition when the whole surface of the body was ciliated, as does the partial ciliation of the foot in certain groups. _Synchaeta_ and many Notommatidae possess a pair of lateral, hollow, ciliated pits on the body, which can be everted to serve as additional swimming organs; these are termed "auricles." The _cuticle_ varies much in texture. It may be smooth and flexible, dotted or shagreened, or in the Loricata firm and of definite shape, constituting a _lorica_, which may be more or less distinctly divided up into areas or separated into distinct pieces. In this case it resists decomposition, and several species are only known by this "skeleton." In _Ploesoma_ it is much thickened and looks like a honeycomb. A regular alternation of harder and softer zones effects the annulation of the body in certain genera. The _hypoderm_ or protoplasmic layer of the skin has no cellular boundaries, though it contains large and distinct nuclei; it is usually somewhat granular. It forms the wall of the body-cavity, which contains a transparent liquid without corpuscles. The principal _external glands_ are the pedal or cement-glands, which secrete a viscid substance that sets in water and serves to anchor the animal. They are formed from an ingrowth of the hypoderm, are usually paired, and open by fine ducts on or near the apex of the toes, when these processes of the foot are present (Fig. 106, _fg_). These glands are mostly absent when there is no foot, as in most Asplanchnidae and in Anuraeidae, but in _Asplanchna herrickii_ a small gland on the ventral side of the cloacal aperture appears to represent the last rudiment of the foot.