Not entirely sure, but they're seen on quite a few early jetliners with turbojets and early low bypass turbofans. May well have been very early noise abatement measures, as the basic aerodynamics of the design should mix cold, and relatively slow, air from around the exhaust with the hot, high pressure gasses coming through it.

Thx guys. 

I've seen changes somewhat similar like on the 787 and 747-8 engines for noise but I never would have guessed with how those looked that those were early attempts at noise abatement. 

As you can see on the Comet, the duct directs cold outside air to shroud the exhaust gas as it leaves the engine. Surrounding the exhaust with a blanket of cold air was on way of reducing the engine note. With the ERJ, as it is a high bypass engine, the exhaust gas is already shrouded, but it is also possible to use the Comet type shroud to direct the cold air in to mix with the hot gas and not just blanket it. This reduces the noise a little more, something that a small business jet or mid size commuter aircraft benefit from.

Matt