vni EXTERNAL FEATURES 201 



scopic in the Bdelloida, both ends being retractile into the 

 middle segment. In most attached, tube-dwelling forms the body 

 is ovate, tapering behind into the elongated stalk-like foot. 



The foot afc the hinder end of the body is usually more or 

 less jointed ; in Pterodina and Brachionus it is long, transversely 

 wrinkled, and retractile. Usually it terminates in a couple of 

 acute, mobile toes, perforated at the tips by the ducts of the 

 pedal glands (Fig. 106, fg), whose viscid secretion serves to 

 anchor the animal. In Botifer there are three of these toes, 

 which are retractile, and in addition there are in this genus, as 

 in most of the Bdelloida, toe-like pointed spurs in pairs on the 

 more proximal joints of the foot. In Callidina the spurs are 

 often perforated, and the toes are replaced by numerous openings 

 on the last joint of the foot (Fig. 109, A) ; while in Discopus the 

 end of the foot expands into a large disc, with numerous pores 

 for the exudation of the pedal cement, and there are no spurs. 

 In Pedalion mirum the foot is represented by two tubular pro- 

 cesses ciliated at the apex and at the outer side near the base 

 (Fig. 117, /). These are inconstant in size and form, that of one 

 side being sometimes reduced or absent, while both are absent 

 in the closely allied species P. fennicum. 



In Melicertidae and Flosculariidae the long foot ends in an 

 expanded disc, which is cupped and ciliated in the larva (Fig. 

 112, B) and in the larva-like male (Fig. 107); but in two 

 species it is prolonged into a long flexible thread which is not 

 contractile. The foot is also, elongated in the Bdelloid genus 

 Actinurus and the Ploimal genus Scaridium. It forms a mere ven- 

 tral disc in Apsilus (and Atrochus ?), and is absent in Asplanchnidae 

 (except Asplanchnopus), Triarthridae, and Anuraeidae, and in the 

 genera Trochosphaera (Melicertaceae) and Pompholyx (Pterodinidae). 



The fringed spines of Triarthridae are jointed appendages 

 moved by powerful muscles ; in Triarthra one is median and 

 ventral, the others being attached to the shoulders. In Polyarthra 

 there are twelve flattened and serrated spines, a bunch of three 

 being attached to the dorsal and ventral faces of either shoulder. 

 An easy transition leads to the hollow appendages of Scirtopoda, 

 which end in a fringe of bristly hairs, themselves feathered with 

 finer hairs (Fig. 117). These processes are in Pedalion six in 

 number, two median (respectively dorsal and ventral), two antero- 

 lateral, and two postero-lateral. As they contain proper muscles, 



