Fig. 242. Diagrammatic section of Cymothoa shewing the dorsal organ. (From Bullar.)
In Cymothoa (Bullar, No. [499]) there appears on the dorsal surface, in the region which afterwards becomes the first thoracic segment, an unpaired linear thickening of the blastoderm. This soon becomes a circular patch, the central part of which is invaginated so as to communicate with the exterior by a narrow opening only ([fig. 242]). It becomes at the same time attached to the inner egg membrane. It retains this condition till the close of larval life.
In Oniscus (Dohrn, No. 500; Bobretzky, No. [498]) there appears very early a dorsal patch of thickened cells. These cells become attached at their edge to the inner egg membrane and gradually separated from the embryo, with which they finally only remain in connection by a hollow column of cells ([fig. 241] A, do). The original patch now gradually spreads over the inner egg membrane, and forms a transverse saddle-shaped band of flattened cells which engirths the embryo on all but the ventral side.
In the Amphipods the epiblast cells remain attached for a small area on the dorsal surface to the first larval skin, when this is formed. This patch of cells, often spoken of as a micropyle apparatus, forms a dorsal organ equivalent to that in Oniscus. A perforation is formed in it at a later period. A perhaps homologous structure is found in the embryos of Euphausia, Cuma, etc.
Fig. 243. Diagrammatic Section of an embryo of Asellus Aquaticus To Shew the Paired Dorsal Organ. (From Bullar; after E. van Beneden.)
In many Branchiopoda a dorsal organ is found. Its development has been studied by Grobben in Moina. It persists in the adult in Branchipus, Limnadia, Estherea, etc.
In the Copepoda a dorsal organ is sometimes found in the embryo; Grobben at any rate believes that he has detected an organ of this nature in the embryo of Cyclops serrulatus.