CLASS ROTIFERA. 191 



times discontinuous, as in Brachionus, or reduced to a few 

 isolated patches, as in Asplanchna. 



It is a question whether the forms with a double band of cilia or those 

 in which it is single represent the more primitive arrangement. It may be 

 supposed that originally there was but a single band which later became 

 double, but it seems more probable that the double condition is the more 

 primitive, from the fact of its frequent occurrence and also since, when 

 a single band is present, it seems to represent in some cases the prge- 

 oral band and in others (Floscidaria) the postoral one. Such a condition 

 of affairs can be most plausibly explained on the assumption that originally 

 two bands were present, and that in some forms the praeoral one gradually 

 gained pre-eminence in its development, the postoral one disappearing pari 

 passu, while in the Flosculariidae the reverse was the case. 



Beueatli the cuticle lies the ectoderm, consisting of a layer 

 of cells whose outlines cannot be distinguished, and within 

 this comes the musculature of the body, which does not, how- 

 ever, form a more or less continuous layer beneath the skin, 

 but consists of aggregations of muscle-fibres into bundles 

 which traverse the body-cavity in various directions, some 

 running longitudinally and forming retractors of the foot and 

 of the trochal disk, while others have a circular direction. The 

 ccelom, in which the muscle-bundles and the various organs lie, 

 is not lined by a special peritoneal layer of cells, but may be 

 traversed by a greater or less number of delicate fibrils 

 arising from amoeboid cells and representing uudifferentiated 

 mesoderm. 



The mouth lies near the ventral. border of the trochal disk, 

 the ciliated bauds serving to produce currents which con- 

 verge toward the mouth-opening, and so carry to it food-parti- 

 cles, which are then carried through the ciliated oesophagus 

 to the pharynx, whose walls contain a somewhat complicated 

 comminuting apparatus, the mastax (Fig. 93, ma), consisting 

 of two calcareous bodies, the mallei, of varying shape, and 

 sometimes also of a median body, the incus. By the action of 

 muscles attached to the mallei, these parts of the apparatus 

 can be brought into contact with each other, and with the 

 incus when this is present, the food-particles being thus 

 comminuted. From the pharynx the food passes through a 

 shorter or longer tube lined with chitiu, which is to be 

 regarded as a continuation of the pharynx, to the stomach, 



