(Order Cladocera):
Anterior opening of digestive tract under labrum; opens into esophagus [Stachowitsch, 1992].
(Order Notostraca):
Anterior opening of digestive tract on underside of head; located under labrum and opening into esophagus [Stachowitsch, 1992].
(Order Diplostraca):
Anterior opening of digestive tract; located under labrum and associated with large preoral cavity (atrium oris). Opens into esophagus [Stachowitsch, 1992].
(Order Anostraca):
Anterior opening of digestive tract on underside of head; located under labrum and opening into esophagus [Stachowitsch, 1992].
(Class Cephalocarida):
Anterior opening of digestive tract; located under labrum and associated with large preoral cavity (atrium oris) [Stachowitsch, 1992].
(Order Cumacea):
Anterior opening of digestive tract. Bordered anteriorly by upper lip (labrum), posteriorly by lower lip (labium); opens into esophagus [Stachowitsch, 1992].
(Order Tanaidacea):
Anterior opening of digestive tract. Bordered anteriorly by upper lip (labrum), posteriorly by lower lip (labium); opens into esophagus [Stachowitsch, 1992].
(Order Decapoda):
Anterior opening of digestive tract on underside of head; bordered anteriorly by upper lip (labrum), posteriorly by lower lip (labium), and flanked by mandibles, maxillules, and maxillae (and to various degrees by maxillipeds). Leads into cardiac stomach via esophagus [Stachowitsch, 1992].
(Order Amphipoda):
Anterior opening of digestive tract on underside of head. Bordered anteriorly by upper lip (labrum), posteriorly by lower lip (labium); opens into esophagus [Stachowitsch, 1992].
(Order Isopoda):
Anterior opening of digestive tract on underside of head (cephalon). Bordered anteriorly by upper lip (labrum), posteriorly by lower lip (labium) [Stachowitsch, 1992].
(Order Mysida):
Anterior opening of digestive tract. Bordered anteriorly by upper lip (labrum), posteriorly by lower lip (labium). Opens into esophagus [Stachowitsch, 1992].
(Order Stomatopoda):
Anterior opening of digestive tract on underside of head (cephalon); bordered anteriorly by labrum, posteriorly by labrum and mouthparts. Opens directly into stomach [Stachowitsch, 1992].
(Order Leptostraca):
Anterior opening of digestive tract on underside of head. Bordered anteriorly by labrum, posteriorly by labium (paragnaths); opens into esophagus [Stachowitsch, 1992].
(Subclass Branchiura):
Anterior opening of digestive tract on underside of cephalothorax; typically enclosed, along with surrounding labrum, mandibles, and paragnaths, in proboscis. Opens into esophagus [Stachowitsch, 1992].
(Subclass Cirripedia):
Anterior opening of digestive tract. Closely associated with labrum and mouthparts (trophy). Opens into esophagus [Stachowitsch, 1992].
(Subclass Mystacocarida):
Anterior opening of digestive tract. Located under labrum; opens into esophagus [Stachowitsch, 1992].
(Class Ostracoda):
Anterior opening of digestive tract on underside of head. Bordered anteriorly by upper lip (labrum), posteriorly by lower lip (labium), and flanked by mandibles (and to a lesser extent by other mouthparts: maxillules, maxillae, maxillipeds). Opens into esophagus [Stachowitsch, 1992].
(Class Ostracoda):
Anterior opening of digestive tract on underside of head. Bordered anteriorly by upper lip (labrum), posteriorly by lower lip (labium), and flanked by mandibles. Opens into esophagus [Stachowitsch, 1992].
(Class Remipedia):
Anterior opening of digestive tract on underside of head (cephalon); located in cavity (atrium oris) formed by projecting posterior margin of labrum and by paragnaths. Perioral area highly setose due to setae of labrum and paragnaths. Opens into foregut [Stachowitsch, 1992].
(Superorder Syncarida):
Anterior opening of digestive tract. Bordered anteriorly by upper lip (labrum), posteriorly by lower lip (paragraphs). Opens into esophagus [Stachowitsch, 1992].
(Order Thermosbaenacea):
Anterior opening of digestive tract. Located on underside of cephalon and bordered anteriorly by upper lip (labrum), posteriorly by lower lip (labium). Opens into esophagus [Stachowitsch, 1992].
(Order Euphausiacea):
Crustacea glossary. Natural History Museum of Los Angeles County. 2011.