The skeletomusculature of a primitive siphonostomatoid copepod, Hyalopontius typicus, is described and compared with that of a relatively advanced siphonostomatoid, Lepeophtheirus pectoralis, an ectoparasite of flatfishes. The comparison is used to assess how adaptation to parasitism has affected the morphology and musculature of the appendages. Anatomy has been related to presumed function wherever possible, and functional interpretations are presented for the feeding apparatus of both species and for a range of other siphonostomatoids. The longitudinal trunk muscles of Hyalopontius are arranged in paired dorsal and ventral bundles, as in free-living copepods, but are smaller and less powerful. A further reduction takes place in Lepeophtheirus, and is presumably correlated with a limited swimming ability. The tagmosis of Hyalopontius is typically podoplean, with the prosome-urosome junction between fourth and fifth pedigerous somites. Hyalopontius possesses a cephalothorax comprising the cephalosome plus the first pedigerous somite. The tagmosis of Lepeophtheirus is modified and the prosome-urosome junction is no longer functional. In adult females the fourth pedigerous somite forms a waist-like region separating anterior cephalothorax from posterior genital complex. The shield-like cephalothorax comprises the cephalosome plus the first three pedigerous somites. The fifth pedigerous, genital and probably the first abdominal somites are fused to form a genital complex. The eudactylinid genus Bariaka is identified as one of the only two siphonostomatoids that possess a free genital somite, not incorporating the first abdominal somite. The structure and musculature of the sternal furca of Lepeophtheirus is described. It is concluded that the furca is an elaboration of the median intersomitic sclerite lying between the maxillipedal and first pedigerous somites. The musculature of the sternal furca is derived from the ventral longitudinal trunk muscles. The musculature of the cephalosomic appendages of siphonostomatoids is reduced compared with that of free-living copepods. The mandibles and maxillules increasingly lose any adduction-abduction movements. In Hyalopontius some adduction of the mandible is possible and extrinsic adductor muscles originating on the anterior cephalic tendon are retained. These are absent from Lepeophtheirus. The maxillules have no ventral extrinsic muscles in Hyalopontius and few dorsal muscles. In Lepeophtheirus there are no maxillulary muscles. The antennae, maxillae and maxillipeds are all basically subchelate and their intrinsic muscles are arranged in antagonistic extensor and flexor groups. These commonly insert on an apophysis that extends proximally from the base of the subchela. A pattern of homologies for the segmental composition of the limbs of the siphonostomatoids is established, based on evidence from their musculature. The inner lobe of the maxillule is homologous with the praecoxal arthrite of other copepods, the outer lobe with the rest of the palp. The proximal segment of the maxilla is the syncoxa. The distal claw represents the basis and its proximal endite, and there is no vestige of the endopod in any siphonostomatoid. The subchela of the maxilliped is derived from the endopod and the endopodal segments are often fused to each other and to the distal claw. The oral cone in a range of siphonostomatoid families is described. Adaptive radiation in feeding mechanisms mainly involves modification of the oral cone and maxillules, rather than of the appendages used for attachment to the host. In Hyalopontius, Pontoeciella and Entomopsyllus the oral cone is specialized for fluid feeding. There are efficient seals both around the distal opening of the cone and along the labrum-labium boundary. Suction pressure is produced by labral muscles concentrated in the proximal part of the cone. In Lepeophtheirus the oral cone is specialized for surface grazing and it lacks efficient sealing mechanisms distally and along the labrum-labium boundary. Food material is transported up the oral cone by sequential contraction of labral muscles. In adult Lernaeocera the oral cone acts as an enlarged oral disc forming a feeding seal distally. Suction is generated by oesophageal peristalsis. The musculature of the swimming legs of Lepeophtheirus is described. Several remotor muscles insert on the ventral body wall remote from the basal foramina of the swimming legs. These muscles probably function as a dorsoventral tensors, and assist in creating suction beneath the disc of the cephalothorax. The posterior sinuses on the rear margin of the cephalothorax function as outlet valves during swimming. They are opened and closed by sinus muscles derived from modified promotor muscles of the second swimming legs.