Sponges

More interesting preadaptations:
This time sponges (wiki).

Quote:

Sponges are among the simplest animals. They lack gastrulated embryos, extracellular digestive cavities, nerves, muscles, tissues, and obvious sensory structures, features possessed by all other animals.

Nice site about sponges.
PIC
Evolutionary history of sponges (Sponges = light blue, Divergence time = yellow)

Choanoflagellates had a lot of the toolkits necessary to develop a nervous system as well as multi-cellularity, even though they are simple uni-cellular organisms that do not form colonial assemblages.

Now the Origin of Nerves are Traced to Sponges

Quote:

Sponges are very primitive animals. They don't have nerves cells (nor muscles nor eyes nor a lot of other things we commonly associate with animals). So scientists figured sponges split from the tree of life before nerves evolved. A new study has surprised researchers, however. "We are pretty confident it was after the sponges split from trunk of the tree of life and sponges went one way and animals developed from the other, that nerves started to form," said Bernie Degnan of the University of Queensland. "What we found in sponges though were the building blocks for nerves, something we never expected to find." In humans and other animals, nerves deliver messages to and from the brain and all the parts of a body. Degnan and colleagues studied a sea sponge called Amphimedon queenslandica. "What we have done is try to find the molecular building blocks of nerves, or what may be called the nerve's ancestor the proto-neuron," Degnan said. They found sets of these genes in sponges.

Nice .
Free, online peer-reviewed article:
A Post-Synaptic Scaffold at the Origin of the Animal Kingdom

Quote:

There are even more fascinating findings from the genome of the sponge. "But what was really cool," he said, "is we took some of these genes and expressed them in frogs and flies and the sponge gene became functional — the sponge gene directed the formation of nerves in these more complex animals. The research, announced this month, was published in the journal Current Biology.

Article with the details:
Article abstract:
Sponge Genes Provide New Insight into the Evolutionary Origin of the Neurogenic Circuit

Quote:

The nerve cell is a eumetazoan (cnidarians and bilaterians) synapomorphy [1]; this cell type is absent in sponges, a more ancient phyletic lineage. Here, we demonstrate that despite lacking neurons, the sponge Amphimedon queenslandica expresses the Notch-Delta signaling system and a proneural basic helix loop helix (bHLH) gene in a manner that resembles the conserved molecular mechanisms of primary neurogenesis in bilaterians. During Amphimedon development, a field of subepithelial cells expresses the Notch receptor, its ligand Delta, and a sponge bHLH gene, AmqbHLH1. Cells that migrate out of this field express AmqDelta1 and give rise to putative sensory cells that populate the larval epithelium. Phylogenetic analysis suggests that AmqbHLH1 is descendent from a single ancestral bHLH gene that later duplicated to produce the atonal/neurogenin-related bHLH gene families, which include most bilaterian proneural genes [2]. By way of functional studies in Xenopus and Drosophila, we demonstrate that AmqbHLH1 has a strong proneural activity in both species with properties displayed by both neurogenin and atonal genes. From these results, we infer that the bilaterian neurogenic circuit, comprising proneural atonal-related bHLH genes coupled with Notch-Delta signaling, was functional in the very first metazoans and was used to generate an ancient sensory cell type.

Whole parts of the nervous system were present in animals that do not have a nervous system, yet these parts are interchangeable and function just like they should in animals that do have a nervous system.