Sand collars are the egg masses of certain sea snails, marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 gastropod mollusks in the family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Naticidae

Naticidae

Naticidae, common name the moon snails, is a family of minute to large-sized predatory sea snails, marine gastropod molluscs in the clade Littorinimorpha.Naticidae is the only family in the superfamily Naticoidea....

, sometimes known as "moon snails".

These egg masses often wash up on sandy beaches, either whole or sometimes in fragments. When they are whole, sand collars are shaped like an old-fashioned detachable shirt or blouse collar

Collar (clothing)

In clothing, a collar is the part of a shirt, dress, coat or blouse that fastens around or frames the neck. Among clothing construction professionals, a collar is differentiated from other necklines such as revers and lapels, by being made from a separate piece of fabric, rather than a folded or...

 (hence the name). They consist of sand grains cemented together by a gelatinous matrix, with embedded eggs.

A fresh sand collar feels stiff and yet flexible, as if it were made out of plastic. Each sand collar contains thousands of capsules, each one housing one or several live embryo

Embryo

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

s. In species with plankton

Plankton

Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

ic development, these embryos hatch out as bilobed veliger

Veliger

A veliger is the planktonic larva of many kinds of marine and freshwater gastropod molluscs, as well as most bivalve mollusks.- Description :...

s. After the eggs hatch, the sand collar disintegrates.