Important as are the functions of the mastax, they are by no means identical throughout the Rotifera, and the general plan of the organ has been very greatly modified in the various series of species ac cording to the requirements of their respec tive habits of life. In itself the mastax is a complicated arrangement of seven princi pal hardened parts (adapted for biting, cut ting, holding and crushing), of powerful muscles, of controlling nerves and tiny glands, all enclosed in a stout-walled cham ber, into whose upper cavity the food is carried. Each hardened part varies greatly in size, shape and relative prominence in the combination, and the parts that are dominant in the mastaces of one series of species may be of secondary importance or even sup pressed, in those of another series. The different forms of the mastax have been grouped under six leading types.
In the malleate type, figured in ventral aspect (fig. 5), the seven hardened parts are all present and of average development. In the centre is the incus, or anvil, comprising the fulcrum, or base (now viewed edgewise), to whose upper portion are hinged two rami, or branches, flattened parts whose free ends, mostly directed upwards, open and shut like shears. In lateral view, the fulcrum appears as a moderately wide plate. It is secured strongly to the mastax wall and has no independent motion. Outside the rami, to right and left and further dorsalwards, are two mallei, or ham mers, each comprising a manubrium, or handle, nearly perpen dicular, and an uncus, or striker, bent sharply inwards towards its fellow, and of ten ending in finger-like teeth. The two rami move in unison, as do also the two mallei, but generally independently of the rami. Only the two unci and the two rami come in contact with the food. This type of mastax obtains among numerous species of the hunting rotifers.
The other types can only be briefly indi cated. In the virgate type, adapted for pumping, the manubria and the fulcrum are both elongate ; the canal being dis tended, and its wall being supported by the hardened parts, a pis ton is supplied by a muscle. In the incudate, or seizing type, the whole incus (but especially the rami), is extremely developed; in the ramate, or bruising type, the unci are divided into several teeth and the fulcrum is reduced. The uncinate, or tearing type, has both manubria and fulcrum greatly reduced, whilst in the forci pate type, having somewhat elongate jaws, the mastax can be thrust forth from the mouth for at least half its length.
Stomach.--From the mastax the food passes next by a usually short oesophagus to the stomach, a fairly capacious organ, occupying in general a large part of the body cavity. It has a stout wall lined with cuticle, mostly ciliated, and on the outside a strong elastic covering. Between these is a dense layer of granular tissue, often divided into conspicuous cells and containing many oil globules. The interior of the stomach is mostly bag-like, but in the bdelloid group it is generally narrowly tubular with a very thick wall and is bag-like only in certain species, known as pellet makers, because, in their case, while the food passes through the oesophagus, it is agglutinated into small pellets before entry into the stomach. Though fairly numerous, such species are mostly
small and dwell in mosses.
In one small group of hunting rotifers the stomach is blind and the undigested residue of food is returned to the mouth for ejectment. In others the stomach is divided into two portions, the lower of which functions as intestine, but usually that organ is a separate dilation of the food canal, following closely behind the stomach, and much smaller, with thinner walls. Thence the residue passes through a short cloaca to the dorsally placed anus, whose position marks generally the hinder limit of the trunk. A pair, or more, of small glands, which secrete digestive juices, are linked to the food canal near its entrance into the stomach.
Among the sessile group the foot commonly ends in a blunt point by which the rotifer can attach itself in any selected position. Among other forms the extremity most frequently carries one, two, or more processes, known as toes, having great variety of form and diversity of function. There are two distinct types, one having a single piece, the other having two, the lower retractile within the upper. The latter type is normal among bdelloid species, which have two, three or four toes, but in some they are re placed by a kind of sucking-disc. Among the hunting rotifers, the toes are always of one piece, sometimes furnished with somewhat claw-like tips. While many have but one, there are generally two, which may be alike in size and form, or very dissimilar. They may be straight, decurved or recurved, short or exceedingly long, slender or stout. Both types are hollow, with perforate tips, from which exudes a viscid secretion brought by tiny ducts from two glands in the upper foot, or lower trunk. By this secretion, the toes, or the sucking-disc, or the blunt point of the sessile foot, are attached to any surface touched, but can be freed at will. Among the bdelloids, the second last segment has two dorsal processes resembling the toes of the hunting rotifers and called spurs to distinguish them. They are mostly short yet vary much in shape and pose.