Light Microscopical and Ultrastructural Organization of Muscles of Priapulus caudatus (Priapulida) and Their Responses to Drugs, with Phylogenetic Remarks

The muscle fibres of the marine invertebrate Priapulus caudatus (Priapulida) are obliquely striated. Thin and thick filaments and characteristic dyad structures were recognized. The ultrastructural organization shows more resemblances with that of arthropod muscles than with that of annelids or molluscs. Pharmacological studies show that the muscles are provided with "primitive" C 16 type of acetylcholine receptors. Tubocurarine, succinylcholine and decamethonium do not block the effect of acetylcholine but potentiate this. This remarkable response may be due to inhibition of choline esterase. No distinct structural or functional differences were observed between somatic and visceral muscles, but the rectum showed evidence of the presence of unspecific amine receptors. The pharmacological responses resemble in the main those known from various simply organized invertebrate phyla. The priapulids, as suggested by Lang in 1953, may te survivors of a very ancient animal group.