CRUSTACEA. 



435 



A 



FIG. 242. DIAGRAMMATIC SECTION OF 

 CYMOTHOA SHEWING THE DORSAL OKGAN. 

 (From Bullar.) 



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 un- 

 paired 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 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 at- 

 tached at their edge to the inner 

 egg membrane and gradually sepa- 

 rated from the embryo, with which 

 they finally only remain in con- 

 nection 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 

 011 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. 



In many Branchiopoda a dorsal organ is found. Its development has 

 been studied by Grob- 

 ben in Moina. It per- 

 sists in the adult in 

 Branchipus, Linmadia, 

 Estherea, etc. 



In the Copepoda a 

 dorsal organ is some- 

 times found in the em- 

 bryo ; Grobben at any 

 rate believes that he 

 has detected an organ 

 of this nature in the 

 embryo of Cyclops ser- 

 rulatus. 



A paired organ 



which appears to be of ^ ^ DIAGRAMMATIC SECTION OK AN EMBRYO OF 



the same nature nas ASELLUS AQDATICUS TO SHEW THE PAIRED DORSAL ORGAN. 



been found in Asellus (From Bullar; after E. van Beneden.) 



and Mysis. 28 2 



