The perithecium is a type of ascocarp which is flask shaped and having an opening called ostiole. It also has well defined wall and the release of the asci occurs through the ostiole. The asci arise from the basal plectenchyma-centrum. The spores are lined with hair like structures known as the periphyses. From the inner walls of the perithecium the asci develops which seen intermixed with the sterile paraphysis. The members of Sphaeriales and Hypocreales produce the fruiting body the Perithecium.