SKIN TUBES 2O5 



a primitive condition when the whole surface of the body was 

 ciliated, as does the partial ciliation of the foot in certain groups. 

 Syncliaeta 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 cellu- 

 lar 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,/^r). 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. 



In addition to these, the ciliated ventral cup below the disc 

 of many Melicertidae secretes a viscid substance (Fig. 116, p) ; 

 and possibly the whole surface of the body is secretory in those 

 species of this group, and of the Flosculariidae, whose tube (Fig. 

 115, A) is uniform and not made of pellets. In several other 

 species belonging to Bdelloida and Ploima-Illoricata a viscid 

 secretion of the surface of the body renders it "sordid" with 

 adherent particles of dirt. 



When the secretion takes the form of a tube, the body can be 

 wholly withdrawn into it by the contraction of the foot. In 

 Floscularia, Stephanoceros, and Conochilus the tube is hyaline and 

 thin-walled ; in Oecistes and Cephalosiphon it is more or less 

 floccose ; and in Limnias it is thin, firm, and annulated. In 

 Melicerta and some species of Oecistes the tube thus secreted by 



