Tubularia sp., a beautiful hydroid found in the low intertidal zone on Bodega Head on 3 August 2012.
The flower-like portion of the polyp is ~4 mm across.
This one caught my eye because it looked so full or swollen. A closer look revealed clusters of small orange structures inside the outer ring of tentacles (see below).
These orange structures are called gonophores (or medusoids in some references) and they're involved in reproduction.
Here's a basic diagram of the Tubularia life cycle. The adult polyp develops gonophores, which either produce sperm (males) or eggs (females). The eggs are retained within the gonophores and develop into actinula larvae (with long tentacles). The actinula emerges from the gonophore and then settles on the bottom. It attaches and undergoes metamorphosis into a juvenile polyp.
Modified from Invertebrates by Brusca and Brusca (2003)
After reviewing the life history, I checked the hydroids again and, Voilà! It was relatively easy to find developing larvae in the gonophores. A couple of days later, I was a little surprised to peer down and see actinula larvae that had emerged from the gonophores (see below).
Now that you know what the actinula larvae look like, you'll be able to see them developing inside the gonophores.
Below is one that's very early. The center gonophore has four long, clear tentacles pointing downward. Above them, you can just make out short orange tentacles of the developing larva inside the gonophore.
The next photo shows a larva in a more advanced stage. Now you can see very long tentacles arching downward inside the gonophore. One rounded tentacle tip is visible at the bottom of one tentacle. (Remember that hydroids are cnidarians, related to anemones and corals, so these tentacle tips are armed with stinging cells).