A rich colony of the cladohepatic nudibranchiate mollusk Embletonia mediterranea was studied from the lagoon region of Cananéia, from March to July (end of summer through winter). To the author's knowledge this species is here redescribed for the first time since its description by Costa (1866), fulfilling some omissions present in the original paper. A comparison is here made with 4 out of the remaining 8 species of the genus. All the species are closely allied, two of the species which were not compared for lack of the necessary bibliography, are probably only a variety of E. pallida which is known to the authors. These are: E. juscata and E. remigata. E. minuta, also not compared, may be a synonim of E. pulchra. The fourth uncompared species: E. pygmex is the only one totally unknown to the authors. The colony became well established in aquaria in S. Paulo where the animals reproduced freely. The whole individual cycle was observed from spawning of a generation to spawning of the following one. The egg masses are lump-shaped, they contain an average of 11 eggs; each full-size animal lays about 16 to 17 eggs in 24 hours. Spawning begins in animals about one third full length. The whole development, from spawning to hatching takes from 48 h to 60 h in temperatures from 20º C to 25º C. Few days after hatching the veligers settle to the bottom and after 24 h to 48 h the young ones abandon the nautiloid shell yet with no cerata neither rhinophores and with reduced cephalic expansions. A comparison is made with the development of E. pallida, described and wonderfully portrayed by Rasmussen (1944). This species is closely associated to Bougainvillia on which were always found numerous egg-masses and adults and on which the animals feed. On the athecate hydroid, are usually found several cauli of Obelia bidentata. Animals reared out of reach of living hydroids had cerata entirely devoid of cnidocysts. In spite of its natural association to the mentioned hydroids, the species is euryphagous since it thrives well in captivity feeding on detritus and perhaps diatoms, small ciliates and suctorians. There is no annual cycle, the species having been found active continuously from summer through the coldest months. No migrations have been recorded and the population seems to be stationary the year round. Finally it was pointed out that this species seems to be a suitable material to study the problems of the relation between the internal medium and the external osmotic pressure.