Larval Development in Discinisca (Inarticulate Brachiopod)

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Abstract
Shell-less Discinisca larvae of 2–3 p.c. (pairs of cirri) and small shelled larvae of 4 p.c. stages, hitherto undescribed, form a growth series with those previously described. The shell first formed during early 3 p.c. or early 4 p.c. stage. In swimming these young larvae did not rotate about their longitudinal body axis, unlike larger larvae. In some larvae pigment granules aggregated in the anterolateral stomach wall, forming “eye spots,” which are not comparable to the sensory eye spots of articulate larvae. The order of appearance of embryonic setae and larval setae was described. The role of the former in floatation and in protective response was suggested. In recent brachiopod ontogeny there is an evolutionary simplification from the presumably primitive condition in lingulids with shelled embryo, shelled larva with statocysts, long planktotrophic existence and well developed trocholophe with continuous budding of cirri to 8-20 pairs; to the discinids with setiferous, shell-less embryo, shelled larva with statocysts, shorter planktotrophic existence and larval trocholophe with a maximum of 4 cirrus pairs; and finally to the articulates with setiferous, shell–less embryo and larva with no statocysts, no differentiated cirri and short free-swimming existence.

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