Final answer:
The phylum Ciliophora is characterized by protists with cilia used for locomotion and feeding, and these cilia are also involved in the excretory systems of certain invertebrates, aiding in the movement and filtration of waste and fluids.
Step-by-step explanation:
The phylum Ciliophora includes protists such as Paramecium and Stentor, characterized by the presence of cilia on their cell surface. These cilia are used for various functions such as locomotion, feeding, and in some cases, capturing prey. For instance, in the excretory systems of planarians and earthworms, the cilia play a crucial role in propelling waste through the tubules; a similar process occurs within the protonephridia of planaria where cilia help in the movement of waste and filtration of fluids. In contrast to the nephridia found in annelids, the human kidney counterpart would be the nephron's structures, such as the tubules where cilia-like structures help in the processing of urine. Notably, fluid in the context of the excretory system could refer to the processed filtrate which is then secreted as urine.