Metamorphic Changes in the Velar Musculature, Head and Shell of some Prosobranch Veligers

Abstract

The velum of Lacuna vincta (Montagu) and Nassarius incrassatus (Ström) is retracted into the shell by 2 dorsal and 2 ventral groups of muscles, bilaterally arranged, which are components of the larval retractor muscle. Each dorsal group passes forward from its area of origin on the shell to the dorsal part of the head where some fibres insert and adjacent ones enter the velar fold. The velum is bilobed in Lacuna and the muscles radiate through the dorsal half of the lobe to insert on the anterior and posterior surfaces and ciliated edge, whereas Nassarius has 4 long velar lobes, 2 dorsal and 2 ventral, and the muscles form a median bundle from which fibres radiate to similar insertions. Each ventral group of muscles passes through the nerve ring lateral to the developing radular sac in Lacuna and to the developing proboscis in Nassarius, and muscles accompanying them insert on the sac and proboscis wall. Anterior to the ring some muscles insert on the lips and the buccal wall posterior to the oral sphincter and alongside attachments of muscles from the foot. The majority pass to the mentum, a median protuberance of the lower lip with non-vibratile cilia, where right and left ventral cephalic retractors and muscles from the foot converge. The pedal muscles attach here and from this area muscles radiate to multiple insertions on the head and ventral half of the velum. This arrangement of the ventral muscles allows the increased angle of mobility required by the ventral areas of the velum in swimming and feeding and the intimate contact between velum and foot.