I drew a redesign of the classic headcrab a week ago and wanted to draw it from the bottom as well because of their weird mouth design but didn't have time to start it back then.

The weird intestine looking organs are probably what hijack the body after the headcrab attaches it self to host and could be responsible for mutating the body. Their absence in the modern games could be why the zombies look more like walking corpses and not as mutated as the zombies in HL1.

The membrane covering the mouth is probably there to protect those organs from the xen environment and those tiny fangs are probably there to inject a paralytic venom into the host and make it harder to detach the crab from the head.