The "dorsal or^an" in Crustacea Malacostraca. 67 



Acanthosoma-stages of five species and in young Mastigopus- 

 specimens of several species, furthermore a rudiment of the same 

 organ in adult specimens. The idea struck me that it must be 

 the so-called "dorsal organ" known in embryos of Crustacea 

 of most orders, but unknown in almost all adult Malacostraca 

 and in larvae of the same sub-class. In examining adults of a 

 good number of species belonging to all orders of Malacostraca, 

 I was surprised that the dorsal organ, though frequently looking 

 only as a rudiment, exists in some or several representatives of 

 most orders. It is found on the surface most frequently nearly 

 vertically above the mandibles or their base. 



II. On our earlier Knowledge. 



Nearh- all original observations on the dorsal organ are 

 scattered in papers very different in contents, though the 

 majority deal with embryolog}^ or development. In 1904 W. T. 

 Caiman's excellent hand-book on Crustacea was published; in 

 1913 W. Giesbrecht's part on Crustacea in Lang's Handbuch 

 der Morphologic was edited. These careful authors had written 

 "records" on the Carcinology during a number of years and 

 were uncommonly well versed in the vast literature; the state- 

 ments of both authors together in the books mentioned on 

 the occurrence of the dorsal organ may therefore be considered 

 nearly sufficient, and an extract is given here. 



On Anaspides (the order Anaspidacea) Caiman writes (p. 

 164) : "On the dorsal surface [of the head], in front of the cervical 

 groove, is a pigmented area with a circular central spot sur- 

 rounded by four minute pits. The significance of this structure 

 is quite unknown, but it may be comparable to an obscure 

 "dorsal organ" apparently glandular in nature, occupying a 

 similar position in certain other Malacostraca". It may be point- 

 ed out, that Caiman in his treatment of the other orders of 

 Malacostraca does not mention the occurrence of a "dorsal 



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