Fig. 240—A simple turbellarian ( Rhabdocœlum). m mouth, sd gullet epithelium, sm gullet muscles, d gastric gut, nc renal canals, nm renal aperture, au eye, na olfactory pit. (Diagram.)
Fig. 241—The same, showing the other organs. g brain, au eye, na olfactory pit, n nerves, h testicles, ma male aperture, fa female aperture, e ovary, f ciliated epiderm. (Diagram.)
Finally, there is a very important new organ in the Turbellaria, which we do not find in the Cryptocœla (Fig. 239) and their gastræad ancestors—the rudimentary nervous system. It consists of a couple of simple cerebral ganglia (Fig. 241 g) and fine nervous fibres that radiate from them; these are partly voluntary nerves (or motor fibres) that go to the thin muscular layer developing under the skin; and partly sensory nerves that proceed to the sense-cells of the ciliated epiderm ( f). Many of the Turbellaria have also special sense-organs; a couple of ciliated smell pits ( na), rudimentary eyes ( au), and, less frequently, auditory vesicles.
On these principles I assume that the oldest and simplest Turbellaria arose from Platodaria, and these directly from bilateral Gastræads. The chief advances were the formation of gonads and nephridia, and of the rudimentary brain. On this hypothesis, which I advanced in 1872 in the first sketch of the gastræa-theory ( Monograph on the Sponges), there is no direct affinity between the Platodes and the Cnidaria.
Next to the ancient stem-group of the Turbellaria come a number of more recent chordonia ancestors, which we class with the Vermalia or Helminthes, the unarticulated worms. These true worms ( Vermes, lately also called Scolecida) are the difficulty or the lumber-room of the zoological classifier, because the various classes have very complicated relations to the lower Platodes on the one hand and the more advanced animals on the other. But if we exclude the Platodes and the Annelids from this stem, we find a fairly satisfactory unity of organisation in the remaining classes. Among these worms we find some important forms that show considerable advance in organisation from the platode to the chordonia stage. Three of these phenomena are particularly instructive: (1) The formation of a true (secondary) body-cavity (cœloma); (2) the formation of a second aperture of the gut, the anus; and (3) the formation of a vascular system. The great majority of the Vermalia have these three features, and they are all wanting in the Platodes; in the rest of the worms at least one or two of them are developed.
Figs. 242 and 243—Chætonotus, a rudimentary vermalian, of the group of Gastrotricha. m mouth, s gullet, d gut, a anus, g brain, n nerves, ss sensory hairs, au eye, ms muscular cells, h skin, f ciliated bands of the ventral surface, nc nephridia, nm their aperture, e ovaries.
Next and very close to the Platodes we have the Ichthydina ( Gastrotricha), little marine and fresh-water worms, about 1/250 to 1/1000 inch long. Zoologists differ as to their position in classification. In my opinion, they approach very close to the Rhabdocœla (Figs. 240, 241), and differ from them chiefly in the possession of an anus at the posterior end (Fig. 242 a). Further, the cilia that cover the whole surface of the Turbellaria are confined in the Gastrotricha to two ciliated bands ( f) on the ventral surface of the oval body, the dorsal surface having bristles. Otherwise the organisation of the two classes is the same. In both the gut consists of a muscular gullet ( s) and a glandular primitive gut ( d). Over the gullet is a double brain (acroganglion, g). At the side of the gut are two serpentine prorenal canals (water-vessels or pronephridia, nc), which open on the ventral side ( nm). Behind are a pair of simple sexual glands or gonads (Fig. 243 e).