In many respects the most remarkable peculiarity of these animals is that, with the possible exception of the excretory organs of the Acanthocephala, there is a complete absence of cilia throughout the whole group. In this respect they resemble the Arthropoda. The universal presence of these small flickering processes of cells from man down to the simplest unicellular organisms makes the absence of these structures most remarkable. In many animals they are the sole organs of locomotion, and in almost all they perform very important functions, both in bringing food and oxygen to the body, and in removing waste matter from it. At present there seems to be no adequate explanation for their absence in the two large groups mentioned above. Nemathelminthes are, with hardly an exception, dioecious—that is to say, their male and female reproductive organs are in different individuals. Their young do not differ markedly from the adults, except in the absence of sexual organs, but the immature stages are usually termed larvae, and not infrequently either inhabit a different host from the adult, or are free when the adults are parasitic, or _vice versâ_. SUB-ORDER I. NEMATODA. ANATOMY.—The Nematode worms, or thread-worms, form by far the largest and most important division of the group Nemathelminthes. The number of species is great, and although the conditions under which they live are of the most varied kind, there is, as a rule, little corresponding difference in structure, and hence the determination of the species is attended with no small difficulty. With few exceptions the shape of the body is filiform (Figs. 66 and 71), the two ends being more or less pointed, and the posterior end of the male, which is generally a smaller animal than the female, is usually slightly recurved. The worms are, as a rule, {125}white, or of the colour of polished ivory; they may be opaque or semi-transparent, but pigment spots are rarely developed. Minute Nematodes abound in moist soil, around the roots of plants, etc., and may easily be detected with the aid of a lens wriggling about amongst the particles of sand and earth. Of the animal parasites perhaps the most familiar is the "round worm" (_Ascaris lumbricoides_, Figs. 66 and 67), which inhabits the alimentary canal of man; others are common in domesticated animals, as _A. mystax_ in the cat and dog, and _A. megalocephala_ in the horse and ox. They are also found living parasitically in plants (Fig. 77), causing the formation of galls and other pathological growths; _Anguillula_ (_Tylenchus_) _tritici_ causes in this way considerable damage to corn, and others attack root-crops, cabbages, etc. The "vinegar eel" (_Anguillula aceti_), which occurs so often in weak vinegar, is another familiar example of this group.