RHIZOTA. 307 



they form an isolated phylum with affinities through Pedalion (Fig. 

 246) to the Arthropoda ; and through the Trochosphere larva to 

 Annelids and Molluscs. As in the Trochosphere, it is impossible 

 to say whether the perivisceral space is a coelom or a haemocoele; 

 probably it is a haemocoele. There is a further important feature 

 of resemblance to the Trochosphere in the form of the excretory 

 organs, which also recall those of Platyhelminthes and Nemertines. 

 The Rotifera are highly specialized animals, and perhaps the most 

 remarkable general fact to note about them is the rarity of marine 

 forms. World-wide in distribution, and extremely rich in species, 

 still they are mainly confined to fresh waters. One cannot help 

 feeling that there must be an equally important marine branch of 

 the phylum which has so far been overlooked, and which when 

 discovered will throw light upon the affinities of the group. 



The jaws or troplii are three in number ; two mallei and one incus (Fig. 

 248, 1). The malleus ms consists of a head or uncus (us), and of a handle or 

 manubrium (mm). The incus (is) consists of two rami (rs), against which the 

 mallei work, and of a basal piece the fulcrum (fm). The modifications (Fig. 

 248, 1-7) of their jaws are as follows : 1. Malleate. Mallei stout, manubria 

 and unci of equal length. 2. Sub-malleate. Mallei slender ; manubria about 

 twice as long as unci. 3. Fordpate. Mallei rod-like ; manubria and fulcrum 

 long ; unci pointed or absent ; rami used as forceps. 4. Incudate. Mallei 

 absent ; rami as curved forceps. 5. Malleo-ramate. Mallei fastened by unci 

 to rami. Unci 3-toothed ; rami large, fulcrum slender. 6. Uncinate. Unci 

 2-toothed ; incus slender. 7. Hamate. Rami, crossed by two or three teeth, 

 fulcrum rudimentary ; manubria absent. 



Order 1. EHIZOTA. 



Fixed when adult, usually inhabiting a gelatinous tube excreted from the skin; 

 foot transversely wrinkled, not retractile within the body, ending in an adhesive 

 disc or cup. 



The foot of the RMzota is unlike the foot of all other Rotifera, in that it 

 is a prolongation of the dorsal, and not of the ventral, region of the body. 



Fam. 1. Floscularidae. Corona* produced into setigerous lobes ; mouth 

 central; velum* a single half circle ventral to the buccal orifice (i.e., the 

 gap in velum is dorsal, Fig. 245, 3) ; trophi uncinate. Floscularia Oken ; 

 Acyclus Leidy, without setae ; Apsilus Metschnikoft', without setae, velum, and 

 foot ; Stephanoceros Ehrbg. , lobes long, convergent ; St. eichhornii Ehbg. 



Fam. 2. Melicertidae.f Corona without setigerous lobes ; mouth lateral ; 

 velum a marginal continuous band, bent on itself at the dorsal surface, so as to 

 encircle the corona twice ; with the mouth between its upper and lower curves, 

 and having also a dorsal gap between its points of flexure (Fig. 245, 1) ; trophi 

 malleo-ramate. Social, the tubes are often adherent to each other. Melicerta 

 Schrank, corona of four lobes, dorsal antenna minute, ventral antennae 



* Corona is used for trochal disc, and velum for ciliary ring or rings, 

 f This family must not be confused with the Leptomedusan sub-family of the 

 same name (p. 134). 



