140 INVERTEBRATE ZOOLOGY 



attachment, though in many forms (Asplanchna, Pedahon, etc.) 

 this is lacking. Individuals of some species are permanently 

 attached. In this instance (Melicerta, Floscularia), they may 

 secrete a tube or may form a case, partly of foreign matter, 

 within which the rotifer can withdraw when disturbed. A few 

 species form colonies by secreting a gelatinous material within 

 which the foot of each individual becomes embedded. In free- 

 swimming forms, the trochal disc is the chief organ of locomotion, 

 though at times the rotifer may loop along like a leech and in some 

 instances outgrowths from the body wall form hollow limblike 

 appendages (as in Pedalion) by means of which the animal skips 

 through the water. 



Trochal Disc. — The plan of ciliation of the trochal disc is 

 usually reducible to various modifications of one or two bands 

 of cilia. In the simplest condition, a single circle of cilia edges 

 the margin of the circular disc. Distortion of this circle at 

 certain points results in the formation of either blunt ciliated 

 lobes (Floscularia) or long ciliated arms (Stephanoceros) but in 

 each of these instances the cilia are arranged in a single continu- 

 ous row. In many instances, a second band of cilia is introduced 

 parallel to the first and in some of these also the ciliated bands 

 may become lobed. Almost always when two rows of cilia are 

 present the mouth occurs between the two on the ventral surface 

 of the disc. In Trochosphaera, which lacks a trochal disc, there 

 is an equatorial preoral circle of cilia with a few cilia postoral 

 in distribution. This condition, as well as the general internal 

 organization of Trochosphaera, corresponds very closely to 

 conditions found in the trochophore larvae of higher inverte- 

 brates. It is because of this close agreement that some contend 

 that rotifers phylogenetically represent trochophore larvae 

 which have attained full sexual development precociously. 



Digestive System. — The digestive system comprises a mouth 

 located on the ventral surface of the trochal disc, an esophagus 

 with its elaborate mastax for triturating food, a stomach, and an 

 intestine which opens near the posterior extremity through an 

 anus. The mastax is highly characteristic of the rotifers but is 

 subject to considerable modifiabilityof form. In many instances, 

 there are three heavily chitinized parts discernable : an incus and 

 two mallei, but either of these elements may be wanting. By 

 action of the muscular esophageal wall, the parts of the mastax 

 are worked together as an effective crushing organ which reduces 



