CRUSTACEA. 



523 



Of the development of the auditory and olfactory organs 

 almost nothing is known. 



Dorsal organ. In a considerable number of the Malacostraca 

 and Branchiopoda a peculiar organ is developed from the epiblast 

 in the anterior dorsal region. This organ has been called the 

 dorsal organ. It appears to be of a glandular nature, and is 

 usually very large in the embryo or larva and disappears in the 

 adult ; but in some Branchiopoda it persists through life. In 

 most cases it is unpaired, but in some instances a paired organ 

 appears to take its place. 



Various views as to its nature have been put forward. There 

 is but little doubt of its being glandular, and it is possible that it 

 is a provisional renal organ, though so far as I know concretions 

 have not yet been found in it. 



Its development has been most fully studied in the Isopoda. 



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 inva- 

 ginated so as to communicate 

 with the exterior by a narrow 

 opening only (fig. 242). It be- 

 comes 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 thick- 

 ened cells. These cells become 

 attached at their edge to the 

 inner egg membrane and gradu- 

 ally separated from the embryo, 

 with which they finally only re- 



FIG. 242. DIAGRAMMATIC SECTION OF 



main 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 



