genetically altered breed of tribbles which do not reproduce but do grow extremely large. (It is later discovered that these are really clusters of tribbles who function as a single tribble, and it is decided that the large numbers of smaller tribbles are preferable to the larger ones.) The Klingons, because of their hatred of tribbles, are eager to get Jones back because he stole a creature they created: a predator called a "glommer" that feeds on tribbles. "Yesteryear" is a time-travel episode in which Mr. Spock uses "The Guardian of Forever", a time gateway from the original series episode "The City on the Edge of Forever", to travel back to his own childhood. This is the only animated Trek episode written by original series and later Next Generation writer D. C. Fontana. This was the first actual appearance of Spock's pet sehlat, first mentioned in "Journey to Babel" and finally named I-Chaya in this episode. One element from Yesteryear that has become canon by depiction within Star Trek: The Original Series is the Vulcan city of ShiKahr, depicted in a background scene wherein Kirk, Spock and McCoy walk across a natural stone bridge (first depicted in Star Trek III: The Search for Spock) in the remastered "Amok Time". Elements of Spock's childhood from "Yesteryear" are also referenced in the Star Trek: The Next Generation episode "Unification" as well as the 2009 Star Trek feature film. Larry Niven's "The Slaver Weapon", adapted from his own short story "The Soft Weapon". It includes some elements from his Known Space mythos such as the Kzinti and the Slavers. This is the only Kirk-era television or movie story in which Kirk did not appear. This episode is also the only animated one in which characters are shown dying or being killed. Casting and voice recording The Animated Series featured most of the original cast voicing their characters. The major exception was the character of Pavel Chekov (Walter Koenig), who did not appear in the series because the series' budget could not afford the complete cast. He was replaced by Lieutenant Arex, whose Edosian species had three arms and three legs; Lieutenant M'Ress, a female Caitian, sometimes stood in for Uhura as communications officer. Besides performing their characters Montgomery Scott and Christine Chapel, James Doohan and Majel Barrett also performed the voices of Arex and M'Ress, respectively. Barrett also voiced various other characters and