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Updated: June 23, 2025
But many facts of embryology and comparative anatomy point to such a form as a very possible ancestor of all forms higher than flat worms, viz., mollusks, arthropods, and vertebrates. It was probably rather long and cylindrical, resembling a small and short earthworm in shape. The skin may have been much like that of turbellaria.
A little higher in the animal world a rude ear has developed, first as a very delicate organ for feeling the waves caused by approaching food or enemies; only later as an organ of hearing. Meanwhile the eye has been developing, to perceive the subtle ether vibrations. The eye of the turbellaria distinguishes only light from darkness, that of the annelid is a true visual organ.
As far as our argument is concerned it makes little difference which of these views we adopt. From our turbellaria, or possibly from some even more primitive ancestor, many lines diverged. And this was to be expected. The coelenterata, as we saw in hydra, had developed rude digestive and reproductive systems.
Notice also that these higher muscles and nerves are here entirely subservient to, and exist for, digestion and reproduction. In the turbellaria the reproductive system has reached a very high grade of development. It is a complex and beautifully constructed organ. The digestive system has also vastly improved; it has its own muscular layers, and often some means of grasping food.
But our little turbellaria, while still without a head, has one end which goes first and can be called the front end. The upper or dorsal surface is usually more colored with pigment cells than the lower or ventral surface, on which is the mouth. It has also a right and left side. It is thus bilateral.
Fresh-water mussels migrate into deeper water at the approach of cold weather. And if the clam has instincts, there is no reason why the turbellaria should not also possess them. But all higher powers develop gradually, and their beginnings usually elude our search. Along the line leading from annelids to insects instinct is becoming dominant.
A little group of these eyes lies directly over the brain, near the front end of the body; the others are distributed around the front or nearly the whole margin of the body. The turbellaria, doubtless, have the sense of smell, although we can discover no special olfactory organ. This sense would seem to be as old as protoplasm itself.
Thus it seems altogether probable that all higher forms are descended from an ancestor of the same general structure and grade of organization as the turbellaria, although probably free swimming, and hence with somewhat different form and development, especially of the muscular system.
In a very few turbellaria we find otolith vesicles. These are little sacks in the skin, lined with neuro-epithelial cells and having in the middle a little concretion of carbonate of lime hung on rather a stiffer hair, like a clapper in a bell. Such organs serve in higher animals as organs of hearing, for the sensory hairs are set in vibration by the sound-waves.
We may picture to ourselves the primitive ancestor of mollusks as a worm having the short and broad form of the turbellaria, but much thicker or deeper vertically. A fuller description can be found in the "Encyclopædia Britannica," Art., Mollusca. It was hemi-ovoid in form. It had apparently the perivisceral cavity and nephridia of the schematic worm, and a circulatory system.
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