Considering that sponges have been around for over 500 million years, possibly even a billion years, many scientists believe they form the base of the evolutionary branch in the tree of life that led to animals. In other words, don’t think of humans as coming from monkeys; we, and every other kind of critter out there, came from sponges, the cousins of the porous yellowy objects you use to scrub yourself in the shower.
A day later Ursula Goodenough on the same blog expands on that theme:
...we eukaryotes (non-bacterial life) trace our ancestry back to a single-celled Most Recent Common Ancestor (MRCA) that inhabited the planet some 1.5 to 2 billion years ago. This MRCA encoded all the core eukaryotic “ideas”: how to make membranes with channels, how to regulate gene expression, how to engage in meiotic sex. These ideas then moved through evolutionary time into numerous radiations, with particular ideas becoming expanded and elaborated, others degraded and lost, in particular lineages.....
she explains further about animal ancestry:
..An ancestral creature with larval globular cells gave rise to two lineages, one leading to modern sponges that have retained the globular-cell idea, and the other leading to modern animals whose “proto-neural” globular cells went on to acquire the capacity to differentiate into full-fledged neurons.
Spot the difference between the two posts?
Maybe the author didn't mean it but the first gives the impression that sponges being the oldest of all animals gave rise to subsequent lineages of animals. But as clarified in the second post, sponges may be the oldest of all animals, but the rest of the animal kingdom did not evolve from them. Rather sponges and other animals share a common ancestor.
Evolutionary diversity forms through a branching process. Oldest simply means that the lineage that gave rise to modern sponges was the first to branch off from the root of the animal tree, root meaning the most recent common ancestor of all animals.
Oldest or "primitive" may also sometimes be taken to mean that the modern creature resembles the MRCA the most. But it certainly does not mean that sponges have stopped evolving since that early divergence. Modern sponges may have conserved certain ancestral traits, for example the globular cells sensitive to stimuli, but may also have acquired several new ones during their long evolutionary history.