In some species the 'cells' are connected by cytoplasmic strands, but there is no evidence of co-ordination between cells. Each probably responds individually to light and mechanical stimulus. You can only make a cell a specialist in sensation, contraction or secretion after you have ensured that it doesn't die of starvation. Choanoflagellates and other multicellular organisms share a series of biochemical and morphological advances which facilitate the condition. We can summarise this a bit .
Lets look at some common features in metazoans.
These are specialised areas of contact between cell membranes. They will also have an adhesive function. These are known from all metazoans, where they are located in synapses. Various RFamides have been found everywhere, acetylcholine in bilateralia only. Acetylcholinesterase,,,, catecholamines and serotonin are found in sponges, but do not seem to have neurotransmitter function.