The nematode cuticle is shown to consist, grossly, of three layers; an outer layer—the cortical layer, an intermediate layerMdashthe matrix layer and an inner layerMdashthe basal layer. With these layers is associated a punctation canal component. The cortical layer carries externally a wholly sclerotized layerMdashthe epi-cortex. The inner part of the basal layer is modified as a basal lamella for the attachment of the hypodermis, from which it is not sharply delimited, and the body musculature. The punctation canal component is most prominent in members of the Chromadorida where it is responsible for all the punctation patterns characteristic of this Order. The system consists of columnar elements in the simplest forms, for example in the Cyatholaimidae, and a series of interlocking transversely fused units in the more complex, for example Euchromadora. In the Desmodoridae and similar “annulate” forms, the punctation canal component forms the major part of the cuticle and appears to be produced by the transverse fusion of punctation canals. In these groups the gross structure of the cuticle is easily established and it is demonstrated that the same system of layering can be identified throughout the Nematoda. It is shown that all modifications of the cuticle can be interpreted as based on and built around a system of punctation canals. The functional requirements of the nematode body are considered and it is demonstrated that the cuticular modifications are developed to meet the necessity for an anisometric cuticle which is flexible longitudinally and strong radially. Thus, the annulate cuticle of the desmodorids and the strongly patterned cuticle of Euchromadora have been developed to meet the functional demands which have been met in forms such as Ascaris, Enoplus, Mermis and Dorylaimus by the development of a series of spirally arranged fibre layers. In many forms the

matrix layer is plasticMdashthat is, easily distortedMdashwhile the cortical and basal layers are fairly rigid although probably elastic. The high degree of organization of the cuticle is shown not only by the massive elaboration occurring in some forms but also by the birefringence of many of the modified component layers. It is argued that the prominent amphids occurring in many of the free-living marine forms act as mechano-stretch receptors. The suggestion is advanced that the punctation canal system evolved originally as a method of allowing the cuticle to grow by the incorporation of additional materials from processes of the hypodermis and it was only later that it became the basis for the many modifications of the cuticle and probably for moulting. The inevitable consequences of the hydrostatic-skeleton: longitudinal muscle system on the nematode body form are discussed and it is stressed that most of the characters considered diagnostic of the Nematoda are functionally determined. Attention is drawn to the opportunities which must result for convergence during the evolution of the group and the difficulties which this introduces in attempting to classify the Nematoda.