Urochordate differentiation is not my specialty, but I am an invertebrate zoologist and can straighten out some things. Narrow down too.
"Planktonic tunicates" is a commonly misused term. The correct application for it is towards salps (thaliaceans), and larvaceans (appendicularians), which really aren't even true tunicates anyway. That honor belongs to the sea squirts (ascideans), which are only planktonic as larvae. Every time I hear the term "planktonic tunicates" I have to check myself from a quick retort, as it's a contradiction in terms.
So far as I know larvaceans are not colonial, and their "houses" are flimsy mucous. The actual animal is a little tadpole-thing wriggling around inside.
Salps come in solitary and colonial forms. They're subdivided into the pyrosomes, salpids ("true" salps), and doliolids. I'm well acquainted with pyrosomes, and the things in that photo don't look like 'em. I'll leave out the technical anatomical jargon.
Salpids and doliolids have a colonial phase comprised of asexual clones. The two groups are best distinguished by their circular muscle bands, "complete" in doliolids and "incomplete" in salpids. Here is where my knowledge fades. I haven't seen enough diagrams or photos of each type to distinguish between the two groups. I can't even identify muscles from your photo, much less see how "complete" they are. On the plus side, most marine biologists still call salps "pelagic tunicates", and don't know enough about marine invertebrates to go any further than what's in a Humann guide. This saddens me constantly.
As for the egg case analogy, that's a good one. Thaliacean integument tends to have this appearance. However, if you actually grabbed the things, you'd soon find out that's all jelly. It's just more organized than the mesoglea folks commonly see in jellyfishes and comb jellies.
I've been going through my texts trying to find a diagram or photo that matches the internal features shown in your photo. I see similarities, but not enough for definitive tagging. A true urochordate or salp specialist would know, 'course I don't know any offhand. Got any more photos?
