Sentence:  The water-vascular system communicates with the exterior by a separate canal through a single pore or a cluster of pores, the hydropore, situated in the so-called <WSD>madreporal</WSD> inter-radius. 
Meaning of the word marked with <WSD> tags:  Pertaining to a sieve-like plate in echinoderms, which admits water into the ambulacral tubes;