306 ZOOLOGY 



SECT. 



the various segments fitting into one another like the tubes of ;i 

 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. 249, 6), this organ is absent. In 

 Pedalion (Fig. 250, 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. 250, ), the resemblance of which to the Nauplius 

 larva of Crustacea is very striking (see Fig. 395), and four genera 

 of unarmoured Ploima, e.g. Polyarthra (Fig. 249, 8) possess simple 

 or fringed setae moved by muscles attached to their bases. 



In the Rhizota the adult is permanently fixed (Fig. 249, 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 retract itself completely, 

 protruding the anterior end with the trochal disc when undis- 

 turbed. In most instances, as for example in Floscidaria (1) and 

 Stephanoceros (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 rnouth. 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 (#), or, as 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 

 the first. The mouth in this case is always placed between the two 

 circlets on the ventral side (Fig. 248), so that the inner or anterior 

 circlet is prae-oral and corresponds with the chief ciliary band 

 of a Trochosphere larva, while the outer or posterior circlet corre- 

 sponds with the post-oral band found in many worm larvae. In the 

 curious globular Trochosphcera (Fig. 250, 3} there is a single 

 equatorial circlet, which is prae-oral, and a few post-oral cilia : here 

 the correspondence with the typical worm larva is singularly 

 close. Lastly both the prae- and post-oral circlets may be pro- 

 duced into more or less complex lobes, as in Melicerta (Fig. 249, 4), 



