Turbellaria

TURBELLARIA (Planarians), a class of flat-worms or Platyhelminthes (q.v.) in which the body is unsegmented and covered with a ciliated epidermis, and an alimentary canal is generally present. Almost all the group are free-living.

General Morphology.

The body is either subcylindrical or flattened and leaf-like or ribbon-like. In length the species range from a fraction of a millimetre to some 20 centimetres. In many forms there is a flat creeping (ventral) surface. At or near the anterior end there may be a pair or more of tentacles, and a pair of eyes, or numerous eye-spots. The margins of the body, in some of the leaf-like aquatic forms, are very mobile and capable of forming fin-like membranes, with which the animals swim after the manner of a skate. The dor sal surface of the body is often pigmented, and the colours and markings, especially in terrestrial forms, may be very striking and brilliant. In certain aquatic species a green or brown colour is produced by the presence in the tissues of symbiotic green or brown algae.

The epidermis consists of a single layer of ciliated, glandular cells. These cells secrete a mu coid slime and often also solid, fusiform, refringent rods known as "rhabdites." Below the epi dermis is a basement membrane, and below this various layers of muscle-fibres (circular, longitud inal and sometimes also diag onal). As in the other classes of Platyhelminthes, the internal or gans are embedded in parenchy matous tissue, though in some of the more highly specialized forms a slight space (schizocoel) sur rounds the alimentary canal.

The mouth is very variable in position. In some forms it is placed anteriorly, but more often it lies towards the middle, or even behind the middle, of the ventral surface. It leads, usually through a muscular pharynx with more or less mobile and pro trusible margins, into a digestive organ which, in some cases (Acoela), is a solid syncytial mass, but more often forms a hollow sac. This sac-like gut may be simple, lobate or branched, but as a rule, has no opening to the exterior except the mouth.

The nervous system consists of paired anterior ganglia lying ventrally to the gut, and giving off various lateral, dorsal and ventral fibres which are interconnected by other fibres. Tactile organs are generally distributed over the skin, and may take the form of special hair-like cilia. Other sensory organs also occur in the form of statocysts, ciliated cephalic pits or grooves, and eyes. The eyes may be mere groups of specialized retinal cells surrounded by pigment, or more complex structures provided with a lens. An excretory system of the platyhelminth type, with flame-cells, is usually present. The external opening may be single or paired, ventral or termi nal, or there may be multiple excretory pores on the dorsal sur face. Almost all the Turbellaria are hermaphrodite, and the re productive organs are usually complex. The ducts of the male and female organs generally open separately, on the ventral surface and towards the posterior end of the body, the male pore being the more anterior.

Classification.

To draw up a natural systematic arrange ment of the Turbellaria is a mat ter of considerable difficulty. An arrangement based primarily on the characters of the digestive apparatus, and due largely to von Graff and Lang, has for many years been looked upon as the standard system, and has been adopted, with slight modifications, in many well-known text-books of zoology. Recently, in conse quence partly of the great increase in the number of known forms, there has been a tendency among specialists to consider this simple classification an unnatural one, and somewhat drastic changes have been proposed, notably by Poche (Arch. f. Naturg., 1926). The older classification, which, up to the present, has been gener ally followed, is given here.

Order I. Rhabdocoelida. Intestine, when present, a simple, straight or irregular sac. Female gonads compact. Small forms (marine or fresh-water) with cylindrical, or more seldom flattened, body.