Tribe 1. Acoela. Digestive organ consists either of scattered cells among the parenchyma or of a syncytial mass without a lumen. Excretory organs of the usual typo absent. A median statocyst present above the brain. Testes follicular. Ovaries paired. Marine forms, including the families Proporidae and Convolutidae.
Tribe 3. Alloeocoela. An irregular or branching intestine present, distinct from the parenchyma, but schizocoel greatly reduced. Excretory organs present. Testes follicular. Vitellaria and ovaries may or may not be distinct. Almost all species marine, but a few in fresh water. Includes the family Plagiostomidae. Order II. Tricladida. Intestine consists typically of three main branches, one median and anterior, two posterior, each of which gives off a series of caeca. Mouth behind middle of body. A single genital pore present. Generally elongate and flattened or sub cylindrical forms, including the families Bdelluridae, Procerodidae (marine), Planariidae (fresh-water), Geoplanidae, Bipaliidae (terres trial) and several others.
Order III. Polycladida. Intestine consists of a central sac giving off laterally a number of branching caeca. Genital pores typically separate. Flattened and generally leaf-like marine forms. Tribe 1. Acotylea. Without a ventral sucker. Mouth at or behind middle of body. Genital pores near posterior end. Tentacles, if present, dorsal. Includes the families Planoceridae, Leptoplanidae, etc.
Mouth at or in front of middle of body, with genital pores just behind it. Tentacles, if present, marginal. Includes the families Pseudoceridae, Euuleptidae, etc.
The group Temnocephaloidea (consisting of Temnocephala and certain other peculiar genera epizoic upon fresh-water cray fish, tortoises, etc.) has sometimes been given the rank of a fourth order, and looked upon as connecting the Turbellaria with the Trematoda (q.v.).
In the classification proposed by Poche, to which reference has been made above, the three main groups are : Acoela (order Proporidea), Rhabdocoeloinei (orders Planariidea, Catenulidea, Rhabdocoela, Temnocephalidea) and Polyclada (order Planocer idea). The Acoela are thus removed from the Rhabdocoelida, and the remainder of the latter, with the Tricladida and Tem nocephaloidea of the old system, included in one large group.
The marine forms usually emerge from their hiding-places only during high tide to feed. These worms creep with a curious glid ing movement, aided by their cilia, or swim actively. Some of the small fresh-water planarians may often be seen creeping up side-down on the surface-film of the water. Many species of tri clads are terrestrial. These occur in damp localities and chiefly in tropical or subtropical countries, though a few are known in temperate latitudes. They are somewhat slug-like in form and habits. A few Turbellaria are parasitic. These are marine, and are found in such animals as sea-urchins, holothurians and molluscs. The free-living forms are almost all carnivorous, feeding either on microscopic organisms or on worms, molluscs and insects. The protrusible pharynx is used either to engulf the prey whole or to pierce it and suck up its juices after it has been enveloped in a coating of mucus.
In the Rhabdocoela the eggs are generally enclosed in shells, each being provided with a number of separate yolk-cells. In the Acoela and Polycladida the ova them selves may contain yolk-granules, and the eggs are usually laid in clumps surrounded by a gela tinous envelope. The ova of tri clads are laid, several together, surrounded by amoeboid yolk cells, in a cocoon, and only a certain proportion of them de velop, the others being used as food-material by the survivors. A form of asexual multiplica tion occurs in certain rhabdocoels (Microstomidae) and triclads. In the former the body becomes constricted in the middle and forms two complete animals, each of which again subdivides, and so on, until a whole chain of in dividuals is formed, head to tail, eventually breaking up into separate organisms. In some of the triclads spontaneous fragmen tation may occur, with regeneration of the parts to form complete animals. But even forms which at one season reproduce in this manner, do so at another by means of eggs. In certain rhabdocoels (e.g., Mesostoma) two distinct types of eggs are produced ac cording to the season. During warm weather the eggs laid are thin shelled "summer eggs" which develop rapidly, while thick-shelled "winter eggs," whose contents remain longer in a resting condi tion, are laid in cold weather. In other cases the eggs are ap parently similar at all seasons, but develop more rapidly in warm water than in cold. Thus in shallow pools subject to sudden and frequent changes of temperature, the population increases rapidly during warm weather, while during cold spells the survival of the race is assured. (See PLATYHELMINTHES.) (H. A. B.)