substructure has recently been studied by @Moeckel2012, who find through $N$-body simulations that those clusters undergo a quick expansion owing fast internal relaxation. Bound exposed clusters are therefore the few survivors of all these processes and represent the remnants of originally more massive ECs.

The observational study of ECs is fundamental to account for most of the newly formed stellar population in the Galaxy and to investigate the interaction with its parent molecular material through stellar feedback. In the past decade, thanks to the development of all-sky infrared imaging surveys, such as 2MASS and GLIMPSE (see Section \[sec:galactic-surveys\]), many new ECs have been discovered in the Galaxy [e.g., @Dutra2003-2mass; @Bica2003-2mass; @Mercer2005; @Borissova2011], significantly increasing the number of known systems. However, so far there have only been a few systematic studies of the whole current sample of ECs and OCs in a significant fraction of the Galactic plane [e.g., @BonattoBica2011; @Kharchenko2012], and none of these studies has distinguished clearly the embedded population from the OC sample (see below). The main goal of this paper is to fill this gap.

Here, we statistically study all OCs and ECs known so far in the inner Galaxy from different cluster catalogs in the literature, 