What is the meaning of Hypostome?

Any of certain mouth appendages of some insects, arachnids, crustaceans, and hydrozoa; in particular:

1. The ventral mouthpart plate in trilobites;
2. The barbed attachment structure associated with the mouthparts of parasitic arachnids (e.g. ticks);
3. The oral tip surrounded by tentacles in hydrozoan cnidarians.

The ventral mouthpart plate in trilobites;

The barbed attachment structure associated with the mouthparts of parasitic arachnids (e.g. ticks);

The oral tip surrounded by tentacles in hydrozoan cnidarians.

Source: wiktionary.org