Excretory organs of different animals are given below. Match the animals with their respective excretory organs.

Animal	Excretory organ/Unit
$(a)$ Balanoglossus	$(1)$ Metanephridia
$(b)$ Leech	$(2)$ Nephridia
$(c)$ Locust	$(3)$ Flame cells
$(d)$ Liver fluke	$(4)$ Absent
$(e)$ Sea urchin	$(5)$ Malpighian tubule
$(f)$ Pila	$(6)$ Proboscis gland

$(A-6, B-2, C-5, D-3, E-4, F-1)$ The correct matching is as follows:
$(a)$ Balanoglossus: $(6)$ Proboscis gland
$(b)$ Leech: $(2)$ Nephridia
$(c)$ Locust: $(5)$ Malpighian tubule
$(d)$ Liver fluke: $(3)$ Flame cells
$(e)$ Sea urchin: $(4)$ Absent (Excretion occurs via water vascular system)
$(f)$ Pila: $(1)$ Metanephridia (specifically, renal organ/Bojanus organ)
Therefore, the correct sequence is: $(a-6, b-2, c-5, d-3, e-4, f-1)$.