Actually, all services are present under /etc/init.d only:

rc0.d contains the services which runs in runlevel 0
rc1.d contains the services which runs in runlevel 1 
rc2.d contains the services which runs in runlevel 2 
rc3.d contains the services which runs in runlevel 3 
rc4.d contains the services which runs in runlevel 4 
rc5.d contains the services which runs in runlevel 5 
rc6.d contains the services which runs in runlevel 6

One more thing, all services are present under rc0.d rc1.d rc2.d rc3.d rc4.d rc5.d rc6.d also, but it is a symbolic link to /etc/init.d only.

See here this is the content of rc1.d directory:

lrwxrwxrwx 1 root root 20 Aug 17 14:54 K15pulseaudio -> ../init.d/pulseaudio 
lrwxrwxrwx 1 root root 22 Nov 28 18:47 K20acpi-support -> ../init.d/acpi-support 
lrwxrwxrwx 1 root root 20 Aug 17 14:54 K20kerneloops -> ../init.d/kerneloops 
lrwxrwxrwx 1 root root 23 Nov 7 15:24 K20openbsd-inetd -> ../init.d/openbsd-inetd 
lrwxrwxrwx 1 root root 15 Aug 17 14:54 K20saned -> ../init.d/saned 
lrwxrwxrwx 1 root root 27 Aug 17 14:54 K20speech-dispatcher -> ../init.d/speech-dispatcher 
-rw-r--r-- 1 root root 369 Apr 14 2012 README 
lrwxrwxrwx 1 root root 19 Aug 17 14:54 S30killprocs -> ../init.d/killprocs 
lrwxrwxrwx 1 root root 19 Aug 17 14:54 S70dns-clean -> ../init.d/dns-clean

Here you can observe the symbolic link to init.d (K15pulseaudio -> ../init.d/pulseaudio).

But here every service is linked to init.d, right? But every service will not start; the reason is two scripts.

The first one is an S script (S30killprocs)---> start

The second one is a k script (K15pulseaudio)---> kill

All the K script services kill the services and all S script services start the services for that runlevel.

In brief

S70dns-clean -> ../init.d/dns-clean start dns-clean service in runlevel 1.

K15pulseaudio -> ../init.d/pulseaudio kills pulseaudio service in runlevel 1.

In short:

ls /etc/rc*.d

This shows you what starts at which runlevel, and within each level the order is determined by the number after the letter (K is Kill, S is start).

PS

you can see all running services by running the following command:

service --status-all