respiratory organs are developed, probably in correlation with the absence of a blood-vascular system. On the other hand, the process of reproduction is elaborately organized. The Planarians are hermaphrodite and, as in so many other small animals, the body, after attaining maturity, becomes in many Planarians practically a genital sac and is soon exhausted by the repeated calls upon its reserves that are involved in the rapid production of eggs and spermatozoa. The intervals between successive clutches has been found in Convoluta roscoffensis to be a month, thus suggesting the influence of the lunar tides upon maturation.
Integument.—The epidermis is ciliated and highly glandular. It consists of a single layer of cubical or oblong cells with the structure seen in fig. 3. The glandular secretion takes various forms, such as mucus, mucinoid granular blocks, or fusiform refringent homogeneous rods. These rods or “rhabdites” are frequently coloured red or yellow, and are highly characteristic of the Turbellaria. Their rea use is unknown. In only two genera does the epidermis produce cuticular spines (Acanthozoon, Enantia) on the surface, but chitinoid hooks, spines and spirals occur frequently on the lining membrane of the male and female copulatory ducts.
Below the epidermis is a firm basement membrane into which the subjacent muscles are inserted. They are divided into outer circular and inner longitudinal groups and subdivided in the larger forms by diagonal fibres, and in the most highly differentiated Planarians there are six muscular layers, two of each kind. In a number of Turbellaria the musculature is modified to form a sucker either single or double and anterior or posterior, and it undergoes further modification in connexion with the pharynx and reproductive organs.
Alimentary Sac.—The alimentary sac consists of a muscular pharynx opening outwards through the mouth and inwards into a median digestive organ which may be solid or hollow, and in the latter case straight, lobate or branched. These characters are correlated with such a number of distinctive features that the classification of the Planarian is based on them. Thus we have the Rhabdocoelida with straight gut and the Tricladida and the Polycladida with triple and multiple branches to the gut. The Rhabdocoelida are further divided into three groups, the Acoela with a simple syncytial gut not sharply separated from the surrounding mesenchyma; the Rhabdocoela, with a hollow gut and a perivisceral schizocoelic span; and the Alloeocoela with a lobate gut and reduced schizocoele. The last group leads one naturally to the Tricladida; the Polyclads being an independent group.
Fig. 8.—The Nervous System of a Simple Planarian (Haplodiscus, one of the Acoela).
B, the brain which gives off a dorsal (DN) and a ventral (VN) plexus and also lateral nerves (LN). The mouth (M) and the otocyst (OT) are shown. The former is ventral, the latter dorsal in position.
The pharynx varies widely in structure. In the Acoela it is a mere thickening and pitting of the integument. In the Rhabdocoela a great number of elaborate modifications are found. These are based on the type of a buccal invagination, which forms the pharyngeal sheath, and from the bottom of this there springs a muscular outwardly directed tube or fold. In the Alloeocoela and Tricladida the pharynx is an elongate protrusible cylinder, and in the Polyclads it may be an immensely distensible frilled organ, the folds of which have independent movement, or an elongate tube. At the base of the pharynx lie the openings of salivary glands. In the Polycladida the section of the alimentary sac into which the pharynx opens is a median stomach from which the intestinal branches radiate. The stomach in few forms is provided with digestive glands. The branches possess an independent musculature and exhibit active peristalsis. The intestine of Planarians is not ciliated, and digestion appears to be largely intracellular and not cavitary.
Mesenchyma.The mesenchyma (Böhmig: parenchyma auctt.) consists of a mass of branched vacuolated cells, imbedded in which lie gland-cells, pigment-cells and the excretory system. It envelops the genital organs, which though in the mesenchyma are not of it, and it forms an investment to the gut and to the space (schizocoel) which often occurs between the gut and the mass of the mesenchyma. The mesenchymatous gland-cells are of different kinds. (1) Single cells in which rods (rhammites) are developed (fig. 4, Rmc). Such cells in embryonic life give rise to a process which perforates the soft basement-membrane and penetrates between the epidermal cells. The process becomes hollow, and the rhammites pass outwards along it on to the surface of the animal, forming in many Turbellarians thickly set rows of rods on the head. (2) Similar cells contain nematocysts in a few Planarians (Microstoma, Stenostoma, Anonymus virilis and Stylochoplana tarda). Whether these