The parallel passage begins in Mark 9:33-37, which reads:

[33] And they came into Capernum. And when they had been in the house he asked them “what were you arguing in the road?” [34] But they were silent: for they have been arguing against each other in the road about who was greater. [35] And sitting he called the twelve and says to them: if you wish to be first, you shall be last of all and servant of all. [36] And taking a child he set it in the middle of them and taking it in his arm he said to them: [37] Who would receive one of these children in my name shall receive me.  And who would receive me, does not receive me but the one who sent me. 

Mark then interjects a passage in vv. 38-41 in which Peter asks about people casting out demons in Jesus name, but who are not actually disciples of Jesus. Jesus then tells Peter that whoever isn’t against them is with them. The parallel continues in vv. 42-48:

[42] And whoever would cause one of these little ones to stumble in faith in me, it is better for him to have a millstone turned by a donkey hung around his neck and to be cast into the sea. [43] And if your hand causes you to stumble, cut it off. It is better to enter into life crooked than while having two hands to depart into Gehenna, into unquenched fire. [45] And if your foot causes you to stumble, chop it off. It is better to enter into life lame than having two feet to be cast into Gehenna. [47] And if your eye would cause you to stumble, pluck it out. It is better for you to enter into the Kingdom of God with one eye than having two eyes to be cast into Gehenna, [48] where their worms would not die and the fire will not be quenched. 

One major distinction between this passage and its parallel in Matthew is that this conversation is presented as a gentle rebuke to the vanity of the disciples, who had been arguing amongst themselves about who was greater.  In Matthew these verses are condensed into the single question “who is greater in the Kingdom of Heaven?”  In both instances, a child is set as the benchmark of humility to aspire to, and the humility of a child in the ancient world is reinforced by the fact that the child is referred to as “it,” like an inanimate object.  There is also an interesting difference between the texts, which will become apparent if we review Matthew 18:3-5, which reads:

[3] and said: truly I say to you, lest you turn and become as the child, you will surely not enter into the Kingdom of Heaven. [4] Whoever therefore humbles himself as this child, this man is the greater in the Kingdom of Heaven. [5] And whoever would receive this one child upon my name, shall receive me. 

Compare this to Mark 9:35-38:

[35] And sitting he called the twelve and says to them: if you wish to be first, you shall be last of all and servant of all. [36] And taking a child he set it in the middle of them and taking it in his arm he said to them: [37] Who would receive one of these children in my name shall receive me.  And who would receive me, does not receive me but the one who sent me.

Here there is a key difference between Matthew and Mark’s understanding of the consequences of a lack of humility.  For Matthew, this precludes the option of entering into the Kingdom of Heaven completely, for Mark this renders you lowly in the Kingdom of Heaven.  Mark also emphasized Jesus as an intermediary for God himself, which Matthew does not include.  

Mark’s language in vv. 43-48 is also interesting to consider, because it quite heavily emphasizes Gehenna.  The language here is slightly different: rather than aeonian fire, we have “unquenched” fire, τὸ πῦρ τὸ ἂσβεστον to pyr to asbeston.  For Matthew, there is an argument to be made that aeonian fire should be translated as “the fire of the age” and indicates a cleansing fire that marks the transition between one epoch and another, here that is not possible, and the word choice here parallels the phrase “and the fire will not be quenched in vv. 48.  

Verse 48 also cites Isaiah 66:24, which reads:

And they shall go out and look at the dead bodies of the people who have rebelled against me; for their worms shall not die, their fire shall not be quenched, and they shall be an abhorrence to all flesh.

NRSV

And they shall depart and they shall see the corpses of the people who transgressed against me. For their worms will not die, and their fire shall not not be quenched, and they shall be a spectacle to all flesh. 

LXX

Here we see that the endless worms and fire are being subjected to corpses, and so the contrast that Mark seems to be implying is between life in the Kingdom of Heaven and the divine punishment of death.  This in turn suggests the possibility that the authors of the Gospels understood their frequent references to punishment in Gehenna of fire as simply the deprivation of eternal life, and corpses tossed into a fire that does not cease.  This understanding is actually quite consistent with the frequent references to eternal life as the reward of the faithful, and I think is perhaps the most defensible reading of that language (although if you are reading the New Testament in translation, Gehenna of fire is most likely translated as “hell of fire” and thus an interpretation is being forced on you by the translators).