This is the scale worm Harmothoe imbricata. Scale worms of the family Polinoidae are Annelids just like earthworms, that is to say they have segmented bodies. H. imbricata has 15 pairs of overlapping scales known as elytra.
On the North East coast they breed during March and April. They come together in pairs; sperm is shed onto the eggs, which are kept under the scales of the female where early development takes place.
Larvae are released into the plankton two weeks later and they will be mature enough to breed the following year. Lifespan is thought to be up to four years.
Another as yet unidentified scale worm. Much smaller than Harmothoe imbracata at 1cm long. The scales do not overlap and they don't cover the entire body. Hopefully someone at the Marine Wildlife of the N. E. Atlantic group will give me a positive ID in due course.
This object was found attached to the underside of a stone quite high up the shore. It has been identified by Richard Lord via Marine Wildlife of the N. E. Atlantic as the egg mass of the sea slug Aeolidia papillosa.
Known as the Common Grey Sea Slug, it feeds on sea anemones and lays eggs at specific times each year. In Guernsey the adults arrive to spawn in March and May and then disappear again. Presumably with Whitby being so much further north, spawning occurs later. I've never seen evidence of sea slugs in this area before.
The above picture shows the egg mass out of the water. The picture below shows the same mass submerged with a 1p piece for size reference.