All life types are actually one element (called "life", hence why "!set type life none" destroys all life on screen just as "!set type life goo" destroys all goo on screen), and the ctype determines which ruleset it follows. Values of ctype between 0 and 23 are valid life types.

Ctype number for each life type:
GOL 0
HLIF 1
ASIM 2
2x2 3
DANI 4
AMOE 5
MOVE 6
PGOL 7
DMOE 8
34 9
LLIF 10
STAN 11
SEED 12
MAZE 13
COAG 14
WALL 15
GNAR 16
REPL 17
MYST 18
LOTE 19
FRG2 20
STAR 21
FROG 22
BRAN 23

Each element is given a unique number. The first 23 are given below. Since ctype is an integer not a string, when you set ctype using the console it converts element names to the numbers below, and sets ctype to the number corresponding to the element name you typed. For example: if you set ctype to "dust", it is set to the number 1 which corresponds to HLIF.
NONE 0
DUST 1
WATR 2
OIL 3
FIRE 4
STNE 5
LAVA 6
GUN 7
NITR 8
CLNE 9
GAS 10
C4 11
GOO 12
ICE 13
METL 14
SPRK 15
SNOW 16
WOOD 17
NEUT 18
PLUT 19
PLNT 20
ACID 21
VOID 22
WTRV 23