Isodiametra vexillaria (Marcus, 1948)

(Fig. 17)

Convoluta vexillaria: Marcus 1948 (p 114); Marcus 1949 (p 13); Marcus 1952 (p 12); Dörjes 1968 (p 84).

Conaperta vexillaria: Antonius 1968 (p 309); Dörjes and Karling 1975 (p 178); Hooge and Tyler 2005 (p 102).

Isodiametra vexillaria: Hooge and Tyler 2005 (p 112); Hooge and Eppinger 2005 (p 8).

Material. Topotype. MZUSP PL. 190, one set of 1.5-µm-thick serial sagittal sections of epoxy-embedded specimen stained with toluidine blue; living specimens in squeeze preparation; whole mounts for fluorescence imaging of musculature.

Locality. Praia de São Francisco, São Sebastião, São Paulo, Brazil, from intertidal coarse sand (23°45’45.1”S, 45°24’27.1”W).

Description. Living specimens ~ 300 m in length and ~ 90 m wide (Fig. 17 A). By transmitted light, body with greenish coloration in digestive syncytium and reddish-orange contents in chordoid vacuoles at margins of body. Rhabdoid glands in distinct rows. Mouth opening on ventral surface, middle of body.

Ovaries and testes paired. Common genital pore opens caudally into male copulatory organ and rostrally into vagina. Vagina surrounded by weakly muscular sphincter; opens anteriorly to walled bursa that leads to bursal nozzle (Fig. 17 B). Long, muscular, glandular penis with small granular gland-secretions in lumen (Fig. 17 B). Penis invaginated into muscular seminal vesicle containing sperm and a cluster of glandular secretions.

Remarks. The specimens of Isodiametra vexillaria collected by Marcus (1948) from Santos Bay, ~ 160 km to the south of our collection site, were considerably larger than ours (500–800 µm long); however, the coloration and shape of the bursal nozzle and penis described by Marcus (1948) matches that of our specimens, and as such, we treat them here as conspecifics.