Why is the arrow on this MOS transistor IC common?
That is the body connection. All MOSFETs have a fourth connection to the substrate, but for discrete MOSFETs this is normally connected internally to the source which results in a parasitic body diode anti-parallel to the source-drain. So in a typical discrete MOSFET you see the body connection tied to the source terminal and you get the MOSFET symbol which you are used to see which looks like this:



simulate this circuit &ndash; Schematic created using CircuitLab
But if you are designing on silicon wafer you get to do whatever you want with the body connection. On a very few discrete transistors, the body connection is run out so you can use it however you want. One such application is a CMOS transmission gate.

Taken from: https://learnabout-electronics.org/Digital/dig55.php
The NMOS body is connected to the most negative rail in the system and the PMOS connects to the most positive rail in the system (much like in your schematic). Doing this allow the MOSFET to block current in both directions which is something it cannot do when a body diode is present. As a result, you can't make a CMOS transmission gate if the transistor body connection is fixed to the source terminal.
I wish more MOSFETs did run the body connection out. There might be other ways or reasons to connect the body, but I don't know them.