What is semantics behind BJT and MOSFET transistor symbols?

BJT:
the arrow represents the base-emitter junction, which acts as a common diode (remember the 0.7 V drop). So the arrow is like the arrow in the diode symbol.

MOSFET:
This dates back from the JFET days: in that case there's a gate-channel P-N junction, and the arrow points from the P to N doped silicon.
The three short lines indicate that it's an enhancement FET: they symbolize that the channel doesn't conduct when the FET isn't activated. In a depletion FET that will be one uninterrupted line, indicating that the channel does conduct when not activated; the FET is switched off by applying a gate voltage.
The line at the gate side symbolizes that the gate lays over the channel, forming a capacitor (a FET's operation is based on capacitance) with it.

In your MOSFET symbol you'll notice that the channel is broken, which means it is an enhancement type of device (meaning off until gate is active - thus the segmented channel). The arrow of the MOSFET indicated the channel/bulk orientation. In a PMOS the channel when active is P-type and the bulk is the Nwell which is N-type. In an NMOS the channel is n-type and the bulk/well is P-type. In diodes the anode is p-type and is the arrow, whereas the cathode is n-type and is a bar. So the channel bulk connection is a junction with the arrow indicating the junction direction.

As an aside, CMOS processes only deal with enhancement devices (with a few exceptions - like native transistors) so in chip design we don't use the broken channel symbol. It's just cleaner.