very very small built thing

Help me out here.  We were wandering down the Altona dog beach trying to retrieve a tennis ball from our dog (isn't it supposed to be the other way?) when we came upon these beautiful air shafts in the sand, presumably a collection of small shell shards and sand glued with a mucous.  After checking online the best I could find is that they are either sandworms or sand worms (Galeolaria) or alien landing sites.  Given that they are quite small, the first option seems unlikely.

If anyone has more information, I'd love to hear from you.