ABSTRACT While working at Naples some years ago, I was led to investigate the early development of the Annelid Eupomatus with a view to determining the origin of the mesoblast bands and their relation to the head-kidneys. This species is common at Naples and breeds throughout the year. The blastulae and gastrulæ are very hardy, and development is normal under the adverse conditions of beat and impure seawater incidental to their study under laboratory conditions. Fertilisation takes place quickly when the ripe generative products are brought together, and material can be easily obtained of any stage. The trochophores can be readily reared to the adult worm in small jars of sea-water to which sufficient food is added from time to time, in the form of cultures of the common Diatom Nitzschia closterium. On this they rapidly grow, and soon attach themselves to the sides of the culture jars and form their tubes.