INTRODUCTION—NEMATODA—ANATOMY—EMBRYOLOGY—CLASSIFICATION—ASCARIDAE—STRONGYLIDAE—TRICHOTRACHELIDAE—FILARIIDAE—MERMITHIDAE—ANGUILLULIDAE—ENOPLIDAE—PARASITISM—NEMATOMORPHA—ANATOMY—CLASSIFICATION—LIFE-HISTORY—ACANTHOCEPHALA—ANATOMY—EMBRYOLOGY—CLASSIFICATION.
The Nemathelminthes include three sub-Orders of very different size and importance. These are—
i. The Nematoda.
ii. The Nematomorpha (Gordiidae).
iii. The Acanthocephala.
Although the members of these groups differ considerably from one another, on the whole there is a closer resemblance between them than between any one of them and any other group of animals, and there is a certain convenience in arranging them under one head.
The following characteristics are common to all three groups of the Nemathelminthes: they are worm-like in form, and with few exceptions are parasitic in the bodies of other animals, either Vertebrate or Invertebrate. Some of them spend their whole existence within the bodies of their hosts, but more commonly they are only parasitic during a certain period of their life; a few, however, lead a free life in water or in damp earth. None of the Nemathelminthes are segmented—that is, their bodies are not divided into a number of parts which serially repeat each other, and which resemble more or less closely the preceding and succeeding parts. They are not provided with any appendages or limbs, but sometimes bear a few bristles or hooks, and in rarer cases suckers. The body, which is elongated and, as a rule, thread-like and tapering at each end, is enclosed in a thick cuticle or hardened secretion of the underlying cells. In no Nemathelminth is there any closed vascular system, nor are special respiratory organs developed.
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.