330 - ZOOLO(iY SEciT. vii 



very long and perfectly retractile. In B. rubens the anterior edge 

 of the lorica is produced dorsally into six spines and is sinuous 

 ventrally. 



3. General Organisation. 



External Characters. — The majority of the Rotifera are free- 

 swimming, being propelled rapidly through the water by the action 

 of the trochal disc. But in the Bdelloida (Fig. 269, 5), in addition 

 to this mode of progression, the animal performs looping move- 

 ments like those of a leech : the tail in this order is freely jointed, 

 the various segments fitting into one another like the tubes of a 

 telescope, and the body is fixed alternately by it and by the anterior 

 end, the trochal disc being kept retracted while the animal moves 

 in this way. Many of the Ploima also have a telescopic tail, but 

 in some, e.g., Asplanchna (Fig. 269, 6), this organ is absent. In 

 Pedalion (Fig. 270, 1) curious skipping movements are performed 

 by the aid of six hollow limbs or appendages, one dorsal, one 

 ventral, and two on each side. These curious organs are ter- 

 minated by feathered setae, and closely resemble the limbs of 

 some of the lower Crustacea : each is moved by two opposing 

 muscles which extend into its cavity {1, B, m). Three pairs of 

 similar appendages are present in the other genus of Scirtopoda, 

 Hexarthra (Fig. 270, S), the resemblance of which to the nauplius 

 larva of Crustacea is very striking (see Fig. 429), and four genera 

 of unarmoured Ploima, e.g. Polyarthra (Fig. 269, S) possess simple 

 or fringed setae moved by muscles attached to their bases. 



In the Rhizota the adult is permanently fixed (Fig. 269, 1-4)- 

 The end of the tail is devoid of the characteristic fork, and is 

 attached to plants or other supports. Moreover the animal is 

 surrounded by a tube into which it can retrace itself completely, 

 protruding the anterior end with the trochal disc 'when undis- 

 turbed. In most instances, as for example in Floscularia {!) and 

 Stej)hanoc(iros {'2), the tube is formed of a delicate, transparent, 

 gelatinous secretion of the epidermis; but in Melicerta (3) it is 

 built up of rounded pellets, which the animal moulds in a cup-like 

 depression on the dorsal surface and places in position one by one. 

 The pellets are usually formed of foreign particles, but in some 

 species are made of the animal's own faeces. 



The ciliation of the trochal disc is subject to considerable 

 variation. In its simplest form the disc is surrounded by a single 

 circlet of cilia, within which lies the mouth. A modification of 

 this type may be produced by the prolongation of the ciliary 

 crown into long arm-like processes fringed with cilia, as in 

 Stephanoceros (:'J), or, a,s in Floscularia (1), into blunt elevations 

 bearing long stiff cilia like those of the Heliozoa. The single 

 circlet may be folded upon itself, or a second type may be pro- 

 duced by the addition of a second circlet within and parallel to 



