2 Gill Internal Morphology**Financially supported by Grant CNRS AI 03 4302.

Publisher This chapter presents an overview of the internal morphology of gills. Each gill arch skeleton is jointed with the posterior skull dorsally and with the copula ventrally. The septum contains nerves and blood vessels and bears the filaments. Two rows of filaments are generally inserted on each gill arch. This whole forms the so-called holobranchs. The mode of the insertion of filaments on the gill arch depends on the morphology of the …