Jones opted for only two scrumhalves, the experienced Ben Youngs and rookie Willi Heinz, for the tournament in Japan starting next month.
Flyhalf Owen Farrell was named as captain for his second World Cup and McConnochie is the only uncapped player in the squad.
“I think it’s always nice to bring new blood in, I think it freshens everything up,” Jones told a news conference in Bristol.
“Ruaridh we’ve been watching quite closely through the club season and were impressed by him. Once he came in the camp he cemented our impression of him, he’s a very good worker and communicator, mature boy, can play a number of positions.”
Eight players named were from Premiership champions Saracens including lock Maro Itoje, number eight Billy Vunipola and prop Mako Vunipola.
Ludlam and hooker Jack Singleton, who won their first caps alongside Heinz in England’s win over Wales in a warm-up match on Sunday, also made the cut with injured former New Zealand under-20 flanker Brad Shields missing out.
“Lewis absolutely came from nowhere,” said Jones of the Northampton player’s meteoric rise.
“We’ve been watching him in club rugby...we thought we’d roll the dice and bring him into camp to see what he could offer and every week he’s just got a little bit better.
“I think we saw yesterday that display, that’s only just the start of his career.”
There was no place for Te’o, who was excluded from the final week of an England training camp after being involved in an altercation with team mate Mike Brown.
Jones, who broke the bad news to those left out on Monday morning, would not give his reasons for the decision.
“We’re not going to discuss why players aren’t selected, because that’s being respectful to them. We’ve had conversations with all of those players about the reasons,” he said.
“We’ve had that conversation with Ben and he understands it. Whether he agrees with it is another matter...he’s just not in our top 31 players at the moment.”
Although teams have until Sept. 8 to submit their squads to World Rugby, Jones decided to name his early.
He said that was the result of experience of previous campaigns.
“We want the squad to know early and now we can get on and be the best prepared England side there has ever been, ready to win the World Cup,” he said.
“We feel for them, they’ve worked hard, and they are excellent players,” he said of those missed out.
“The only message we have for them is that they’ve still got to be ready because the circumstances of a World Cup dictates that if you have injuries then they could get their opportunity later down the track.”
Jones said he would have liked to take three of every specialist position but that would have left the squad short elsewhere.
England play three more warm-up matches – away to Wales and home to Ireland and Italy – before heading to Japan, where they kick off their campaign against Tonga in Pool C on Sept. 22.