First, why stop with verse 19?
The hills bring him their produce,
and all the wild animals play nearby.
Under the lotus plants he lies,
hidden among the reeds in the marsh.
The lotuses conceal him in their shadow;
the poplars by the stream surround him.
When the river rages, he is not alarmed;
he is secure, though the Jordan should surge against his mouth.
Can anyone capture him by the eyes,
or trap him and pierce his nose? (Job 40:20-24, NIV)
While this is a large creature that dwells mainly in water, it can be hidden by reeds and concealed by lotuses. In order to reconcile its status as a large animal with being covered by reeds, it seems as though this creature is often mainly submerged in water. Sounds quite a bit like a hippo.
As is typical of Hebrew poetry, the description of the behemoth consists of couplets: two lines that are thematically linked together. In each couplet, the second line generally re-states or expands in a complementary fashion on what the first line says.
Verse 16 says, "Lo now, his strength is in his loins, / and his force is in the navel of his belly." Both halves deal with strength, and "loins" and "belly" are closely related (loins either refers to the pubic region or hips, such as Genesis 37:34 and Exodus 28:42, or to the centre of a person's strength, such as Proverbs 31:17 and Nahum 2:1).
Verse 18 says "His bones are as strong pieces of brass; / his bones are like bars of iron." Again, the symmetry between the halves of the verse is obvious. So, we can expect that verse 17 also has this form, and since the verses on each side of it deal with strength, we can expect verse 17 to deal with strength as well.
Here's what it says in the KJV (I've again added a slash to divide the lines): "He moveth his tail like a cedar: / the sinews of his stones are wrapped together." There's two words that are interesting. First, the Hebrew word translated in the KJV as "stones" only occurs here in the Bible, so its meaning is speculative. In the KJV and other early English translations, it is rendered as "stones", a word which means "testicles" when it refers to a body part (see Leviticus 21:20 and Deuteronomy 23:1 for other examples of this usage). In the Latin Vulgate, this word is translated "testiculorum". In newer translations, it is often translated as "thighs" instead, although there's not much reason for this change, aside from the fact that it's less likely to induce giggles when read aloud from the pulpit. It appears quite likely that this word indeed refers to the creature's testicles.
The second interesting word is "moveth", which in Hebrew is the word chaphets. While this Hebrew word occurs 75 times in the Bible, it is only translated as "moveth" here. Every other time it is translated as either "delight", "please", "pleasure" or variations on that. In the Greek Septuagint, this word is translated as histemi, a word with a meaning that includes "to cause to make stand", "to make firm" and "to uphold or sustain".
Why don't translators consistently render the word in this instance and instead use a nearly opposite meaning, "moveth"? Perhaps because it doesn't seem to make a lot of sense otherwise -- unless you put all the pieces together. Verses 16-18 all describe the strength of the behemoth. Verse 16 ends with describing the force it has in its loins. In the next verse, the second half talks about tightly wrapped testicles, and we can expect that the first half somehow complements this thought. It refers to a "tail" that is delighted or pleasured so it is as firm, upheld and sustained as a cedar tree.
What ever could it be referring to?
And, how could this fit into the purpose of God's speech, which was to take Job down a few notches by showing wonders in God's creation that surpass or mystify Job in various ways? Is it at all relevant that male hippos happen to be one of the most well-endowed land animals?
I wonder. I know I'll never cease to be humbled by Job 40:17. I mean... like a cedar tree! Wow.