In UJTs (case d) it is the one between the emitter and the bulk semiconductor which connects the two base terminals.Note that when the substrate terminal is not shown in the symbol, the arrow is placed on the source and will point in the opposite direction, since the source has the same semiconductor polarity of the channel, which is always opposite to the substrate polarity: N-channel devices have P-type substrate and viceversa. In MOSFETs it is the one which exists between the channel and the substrate (the terminal where the arrow is placed in the symbols you posted), which is not available externally.In JFETs it is the one between gate and channel.In BJTs the PN junction is the base-emitter one.

In short, the arrows show the current direction of a PN junction when forward biased. I would not be surprised if the arrow direction convention follows the order of the PN junction (wasn't immediately obvious to me which type the example structure on Wikipedia was for). I've never used unijunction transistors (case d), but looking at the wikipedia page shows a similar doping structure as the JFET, the only difference the lack of an insulated gate (names also have changed, apparently it follows the 'BJT' type naming of base and emitter). The P-channel JFET is the opposite so the arrow points out of the gate. For an N-channel, the gate is P-doped and the body is N-doped, so the arrow points from the gate in. Here, the PN junction is between the gate and 'body' (semiconductor section connecting the drain and source I don't know what the correct for this part of a JFET is so I just called it the body because it takes the bulk volume of the JFET). Your symbols for b (FET) are JFET symbols. I have no good explanation for why these are this way, though I suspect it might follow a similar pattern and the semiconductor topology is different from 'traditional' MOSFET topologies. Interestingly, Wikipedia has symbols for 'MOSFETS with no bulk/body' which have opposite arrow directions. Likewise, a P-channel MOSFET has the reverse condition. For an N-channel MOSFET, the source is N-doped and the body is P-doped, so the arrow points from the source to the body. In a MOSFET, the body is often connected to the source. The PN junction (between base and emitter) goes from the center out. An NPN has stacked N, P, and N doped channels. The arrow indicates the order of the junction (base to emitter or emitter to base). For BJT's there is a PN junction between the base and emitter.

Pnp symbol transistor