VTT PHYLUM TROCHELMINTHES 323 



Systematic Position of the Example. 



Brachionus rubens is one of the several species of the genus 

 Brachionus : it belongs to the family Brachionidce, and to the 

 sub-order Loricata of the order Plo'ima. 



It is placed in the order Plo'ima in virtue of its active swimming 

 habits and the absence of looping or skipping movements. The 

 presence of a distinct lorica places it in the sub-order Loricata. 

 The family Brachionidae is distinguished by having a box-like 

 lorica open at both ends, and a long, flexible, retractile tail with 

 wrinkled surface and forceps-like termination. In the genus 

 Brachionus the lorica is not marked with ridges, and the tail is 

 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. 274, 5), in 

 addition to this mode of progression, the animal performs looping 

 movements 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. 274, 6), this organ is absent. In 

 Pedalion (Fig. 275, 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. 275, 2), the resemblance of which to the nauplius 

 larva of Crustacea is very striking ; and four genera of unarmoured 

 Plo'ima, e.g. Polyarthm (Fig. 274, 8), possess simple or fringed setae 

 moved by muscles attached to their bases. 



In the Rhizota the adult is permanently fixed (Fig. 274, 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 Floscularia (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 



Y2 



