The starfish is pretty cool. Nothing too difficult but its fairly unique - at least as far as the organisms we've seen. The thing has no centrail brain - like all other echinoderms.
The gonads - like most other echinoderms, they're grainy and there are lots. There's no way of determining between male and female gonads with the naked eye.
The hepatic caeca - they're used to absorb food and secrete enzymes. They function like livers, hence the term, "hepatic". "Hepatic" always refers to the liver.
The ring canal is the main pump for the starfish....water gets distributed from there. The mouth is in the dead center of it and on the other side.
The ampullae are also pumps - they contract and water is expelled. They are sort of like ballast tanks for the starfish.
The ambulacral ossicles are just what their name implies - "ossicle" = "little bone", which is what they are. Little plates that cover the radial canal.