Fine structure of the larvae of the Myzostoma cirriferum (Myzostomida)

Adults of Myzostoma cirriferum are obligate ectocommensals that live on the surface of comatulids of the genus Antedon from the North East Atlantic and Mediterranean. M. cirriferum is a simultaneous hermaphrodite which reproduces by transferring spermatophores that penetrate the integument of the receiver myzostomes. Fertilized eggs are shed into the water column where they divide. After the embryonic stage that lasts 24h, the motile individuals enter into the larval stage. Three types of larvae are recognized in M. cirriferum: the spherical protrochophora (occuring between 24 and 48h after the laying), the pyriform trochophora (48-72h), and the pyriform, setigerous metatrochophora (72h until metamorphosis). The body of the protrochophora and trochophora is made of undifferenciated cells and ciliated cells. Two types of undifferentiated cells are present: the first contact the thin cuticle, have microvilli and include a lot of yolk droplets; the second are internal and have no or only a few yolk droplets. No basal lamina has been observed in the body of these larvae. Huge tissular modifications occur in the young metatrochophora (i.e. 96-144h after laying). The epidermis is now well underlined by a basal lamina. Two pairs of 4 chaetae lie on the sides of the larval body and a sensory caudal appendage is differentiating at its end. The digestive system is forming. It is composed of a pharynx included in an extensible introvert and a closed, internal digestive cavity. The nervous system appears and is composed of an epispheral nervous mass, a peripharyngeal collar, nervous processes and sensory cells. The anatomy of the late metatrochophora stays unchanged until metamorphosis, which generally occurs 7 days after laying. The body of the larva is made of twelve cell types. Seven are ectodermal: covering cells and ciliated cells make up most of the epidermis; primary chaetoblasts and lateral cells form the chaetal follicles; secondary chaetoblasts will build the chaetae of juveniles after metamorphosis; pharyngeal cells and myoepithelial cells make up the pharyngeal epithelium. The other cell types are neurodermal (neurons and sensory cells), endodermal (the digestive cells that border the digestive cavity) and mesodermal (parenchyma cells and muscle cells