Excretion and Osmoregulation in the Flatworms

Field: Agricultural and Biological Sciences (Life Sciences)

In flatworms, excretion is accomplished by the epithelial lining of the gut, by the body wall, by exocytosis of vesicles, and by protonephridia. Osmoregulation in flatworms has not been demonstrated conclusively, but is thought to take place in the protonephridia. There are two types of protonephridia described from flatworms, a singlecell type (composed of a blindly ending tube of cytoplasm) and a two-cell type (composed of a terminal cell that interdigitates with a proximal tubule cell). Numerous structures associated with the protonephridium have been used to study phylogenetic relationships among the flatworms. The Catenulida are the only flatworms that have an unpaired protonephridial system and a unique terminal cell type. The Macrostomida and the Proseriata have a two-cell protonephridium with processes from the terminal cell external to those of the proximal tubule cell. The Prolecithophora, Lecithoepitheliata, Typhloplanoida, Kalyptorhyncha, Dalyellioida, Temnocephala, and Tricladida have a single-cell protonephridium. The Neodermata also have a two-cell protonephridium, but the processes from the terminal cell are internal to those of the proximal tubule cell. Based on protonephridial characteristics, the Neodermata appear to be more closely related to the Macrostomida and the Proseriata than to the Rhabdocoela and the Lecithoepitheliata. Excretion has been defined as the aggregate of processes directed towards the regulation of the internal environment of an organism by removing from it, or rendering harmless, injurious, superfluous, or unnecessary substances (Beklemishev, 1969, p. 327). This includes: (1) removal of waste products of metabolism; (2) regulation of osmotic pressure; (3) regulation of ionic composition; and (4) removal of unnecessary or harmful substances. In flatworms, excretion is thought to be accomplished by the epithelial lining of the gut (Meixner, 1938; Westblad, 1923), by the body wall, by exocytosis of vesicles such as some of those found in the excretory system, and by the protonephridial system. The protonephridial system also may function in an osmoregulatory capacity, although evidence for this role is not conclusive. A protonephridial system is present in most flatworms with the exception of the Nemertodermatida and the Acoela (see Ehlers, 1985). Within these two groups, a localized excretion is thought to take place through the digestive syncytium (Beklemishev, 1969) and the peripheral parenchyma in the form of storage kidneys (Westblad, 1923). Protonephridia of most invertebrates consists of a tube or tubules that open distally at the surface and end proximally in one or more terminal structures. According to Wilson & Webster (1974), there are three types of terminal structures: (1) flame cell with a nucleated cap that bears a tuft of cilia surrounded by a tubular barrel of cytoplasm (this type is composed of a terminal cell that interdigitates with a neighboring tubule cell); (2) flame cell composed of a ' Based on a paper presented at the symposium entitled Biology of the Flatworms, sponsored by the American Microscopical Society and convened 29 December 1991 in Atlanta, Georgia. TRANS. AM. MICROSC. SOC., 112(1): 10-17. 1993. ? Copyright, 1993, by the American Microscopical Society, Inc. This content downloaded from 207.46.13.114 on Sun, 28 Aug 2016 04:29:31 UTC All use subject to http://about.jstor.org/terms VOL. 112, NO. 1, JANUARY 1993 blindly ending tube of cytoplasm within which lies a tuft of cilia (this type is composed of a single cell that comprises both the ciliary tuft as well as the tubule and may or may not have a terminal nucleus; and (3) solenocytes, which are elongate tubular cells with a nucleus in a cap of cytoplasm and with a single flagellum lying in the lumen. The first two types are common in flatworms and will be the focus of this review; solenocytes have not been reported from flatworms. In turn, three types of cells are associated with the protonephridia of flatworms, namely (1) cyrtocytes (the multiciliated terminal cells found at the proximal end of the flame cells; Brandenburg, 1974), (2) paranephrocytes (lacunae-filled cells found in close proximity to the tubules; these appear to possess excretory capabilities; Reisinger, 1968), and (3) tubule cells (cells that form the proximal and distal tubules; in turn, there appear to be at least three types of tubules according to Wilson & Webster, 1974): (a) those tubules that are formed by a single cell wrapping around itself and sealing one layer to another by a septate junction; (b) those tubules that are composed of two or more cells with distinct membranes that are joined together by septate junctions; and (c) those tubules that are formed in syncytial fashion with more than one cell but lack septate junctions. Generally, the terminal cell is rich with mitochondria and vesicles. In a twocell type of protonephridium, processes from the terminal cell project distally and interdigitate partially with processes from the proximal tubule cell to form the weir. The weir is the filtering apparatus of the protonephridium. Between the array of processes (rods), there is generally a thin membrane. When the cilia beat in the lumen of the weir, a pressure gradient is formed drawing fluid across the membrane into the lumen of the weir (Ruppert & Smith, 1988). The rods commonly contain microtubules thought to function in the support of the weir (Rieger et al., 1990). Between the cilia is electron-dense material that may function to band the cilia together (Wilson & Webster, 1974). Tufts of cilia vary in number from two in members of the turbellarian order Catenulida (see Ehlers, 1985) to 120-135 in the trematode Fasciola hepatica (see Pantelouris & Threadgold, 1963). Although the number of cilia is consistently two in the catenulids, the number is not consistent among flame cells of other flatworms, varying even within an individual worm. The cilia may bear distinct rootlets, rudimentary rootlets, or no rootlets at all. In some flatworms, there are finger-like projections from the rods called leptotriches (Kiimmel, 1962). Leptotriches may be external or internal to the weir. Although their function has not been demonstrated, external leptotriches are thought to keep neighboring cells away from the weir, and internal leptotriches are thought to assist in keeping the wall of the weir away from the ciliary tuft (Wilson & Webster, 1974). Both types seem to increase filtration. Internal leptotriches also may possess microvilli to further increase their surface