The gut is usually direct in its passage through the body and is coiled in only a few water fleas of the order Anomopoda. The fore gut shows the greatest range of structure; in some crustacean species it is a simple tube, but in decapods it reaches great complexity in forming a chitinized structure called the gastric mill. This consists of a series of calcified plates, or ossicles, that are moved against each other by powerful muscles, making an efficient grinding apparatus. The junction between the mill and the mid gut is guarded by a filter of setae, which prevent particles from passing into the mid gut until they have been degraded into a sufficiently small size. The structure of the mid gut is also variable among species but generally has one or more diverticula, or pouches, which are involved in various digestive processes. These diverticula may be simple, as in Daphnia, or complex and glandular, as in the decapods. The hind gut is usually relatively short and lined with cuticle. The exit is controlled by a muscular anus, which in some forms had dilator muscles that control anal swallowing.