ROTATORIA OF THE UNITED STATES. 
289 
Mast-ax. — The mastax is a muscular, pharynx-like structure, forming the first 
part of the alimentary canal (mx., pi. vii, fig. 63). Its anterior end forms a nearly 
circular area on the corona, within which lies the mouth (pi. hi, fig. 31, m). The 
two curves of cilia above spoken of, at the sides of the mouth, are really borne on 
the end of the mastax, as appears when the latter is pushed far out (pi. vi, fig. 56). 
The mastax is large, filling up a considerable portion of the anterior part of the 
lorica. It is composed chiefly of a mass of muscles, which act upon the ehitinous 
jaws. Only the anterior part of the mastax is hollow and receives the food, the 
posterior three-fourths or more being a solid mass of muscle. The transverse 
muscles are often very evident as striations (pi. xui, figs. 108, 111, 115, 119). 
In consequence of the asymmetry of the trophi the mastax frequently shows an 
unsymmetrical form, as, for example, in fig. 61, pi. vi. The oesophagus opens into 
the mastax on its dorsal side near its anterior part, as shown in fig. 63, pi. vn, and 
fig. 105, pi. xii. In many species the oesophageal opening is clearly somewhat on 
the right side of the mastax. 
The mastax frequently has connected with it one or more prominent glands. 
One on the left side is especially marked in Rattulus muliicrinis Kellicott (pi. vi, 
fig. 57) and Rattulus latus Jennings (pi. vn, fig. 65). These glands are apparently 
not present in all species. 
Trophi. — The ehitinous jaws or trophi vary a great deal among the different 
species, and usually show a considerable degree of asymmetry. The trophi of 
Rattulus carinatus Lamarck were well described by Gosse (1856) in his classical 
paper on the “Structure, Functions, and Homologies of the Manducatory Organs in 
the class Rotifera.” The trophi of Rattulus longiseta Sclirank (pi. vm, figs. 71, 72) 
f urnish a good example of the typical structure. Following Gosse, we may distinguish 
three main portions — the two lateral parts, known as mallei , and a central structure, 
the incus. Each of these is composed of several portions. 
The malleus consists of two chief parts, a long distal rod, the manubrium (mu.), 
and a shorter proximal portion, the uncus (u.). The two mallei are unequal in size, 
the left one being the larger. The left uncus bears teeth, while the right one is 
merely a straight rod without teeth. 
The incus or central portion consists of three main parts. There is a long 
curved median rod, the fulcrum ( fu . ), which, as the side view (fig. 71) shows, lies at. 
a level nearer the ventral surface than do the manubria. In side view the fulcrum is 
seen to consist of two rods, the ventral one being very thin and united to the other 
by membrane. The fulcrum bears at its proximal end two large structures known 
as rami (ra.). These articulate with the fulcrum and inclose a space between them. 
At their proximal ends they, like the unci, bear a number of teeth. The rami have 
their lower or distal ends produced into a. long process for the attachment of 
muscles. These, with Gosse, we may designate as the alulae (at.). The left-alula is 
considerably longer than the right. 
In addition to these chief portions there are a number of ehitinous rods forming a 
framework which lies on the dorsal side of the proximal end of the trophi (pi. vm, 
fig. 71, su.). These arise from the manubria and are connected with the rami. The 
function of this framework is not. very clear. In some cases it seems to support a 
sort of ehitinous fringe about the mouth (pi. xm, fig. 118). 
F. C. B. 1902—19. 
