The larval musculature of the barnacle Ibla quadrivalvis Cuvier (Cirripedia, Lepadomorpha)

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Abstract
The lecithotrophic development of Ibla quadrivalvis comprises six naupliar stages and a cypris stage. Despite lecithotrophy, the planktonic nauplius has a complete skeletal musculature, associated in part with a ventral endoskeleton. The naupliar musculature remains unchanged for the first five naupliar stages. Modifications of the musculature begin to occur in stage VI, preliminary to the change at the sixth moult to the musculature of the cyprid. In this transition, some naupliar muscles are histolysed, some cypris muscles arise as new developments and some muscles are transformed from naupliar muscles to cypris muscles. Details are given of these changes. The pattern of muscle development during the change from nauplius to cyprid in I. quadrivalvis is compared with the more extreme histolysis and replacement of the musculature during the nauplius-cyprid moult of Semibalanus balanoides. The cypris musculature is also compared with that of the rhizocephalan Lernaeodiscus porcellanae. In spite of lecithotrophy, I. quadrivalvis retains a relatively generalized arrangement of thoracican naupliar muscles and cypris muscles and a generalized transition from one to the other. The evolution of total muscle replacement at the nauplius-cyprid moult in advanced thoracicans is discussed.