Ph.D. / The Branchiura is a group of ectoparasitic crustaceans which infect mostly fishes. The subclass contains four genera including Argulus, Chonopeltis, Dolops and Dipteropeltis. Despite their reputation as pests our understanding of their biology is meager. In the first part of the study the anatomy and physiology is described. The species studied include Argulus japonicus, Dolops ranarum and Chonopeltis australis. Live specimens of A. japonicus, D. ranarum and Chonopeltis australis were collected, fixed in Todd's fixative and processed for transmission electron microscopy. The results gleaned from A. japonicus and D. ranarum indicated that the ultrastructure and physiology of the cells were similar to the digestive epithelia of free-living copepods. The anterior midgut consists of Resorptive cells (R cells) and the enteral diverticula consist of two cell types including R and Fibril cells (F). The R cells contain lipid droplets and F cells contain rough endoplasmic reticulum indicating that the absorption of nutrients and digestion occurs in the anterior midgut and enteral diverticula. The lipid droplets in both species are large in size and number. Blister cells/B cells and R' (apostrophe) cells occur in the posterior midgut and are involved in the processing of digestive waste. The results from C. australis were different from A. japonicus and D. ranarum and reveal the possible presence of F/R cells with multiple functions of both absorption and enzyme secretion. The cells contain few lipid droplets and instead contain many hexagonal-shaped crystalline structures that are interpreted to be proteinaceous. The differences in diet are thought to be influenced by morphological differences in the feeding appendages. The second part of the study involved the supraoesophageal ganglion or brain of C. australis. This genus is unique among the Crustacea since the antennules are absent. This trait poses implications regarding the brain morphology since particular parts of the brain impart nerves that innervate certain appendages in arthropods. Specifically the deutocerebrum or midbrain innervates the antennules. A study was conducted to discover whether the antennular nerves were absent. Specimens of C. australis were fixed in AFA, sectioned and stained with Heidenhain's AZAN. Results showed that the antennular nerves were absent and the deutocerebrum was smaller in comparison to the other brain segments indicating degeneration. Moreover, C. australis possesses a reduced number of sensory sensilla on its external surface compared to related genera. The loss of antennules does not affect its sensory capabilities in host searching. However, the reduced numbers of sensilla and its feeble swimming capabilities are more likely to reduce its efficiency in host searching.