68 Studies on Arthropoda. I. 



organ" in the adult of any other form, but possibly he refers 

 in the lines quoted to the organ found in embryos of other 

 orders. — On the Tanaidacea he says (p. 194) : "A paired "dorsal 

 organ" is present" in the embryo; on p. 213: "A "dorsal organ" 

 is present in many Isopod embrj'os, and assumes very different 

 forms", which are mentioned. On the Amphipoda (p. 237) : 

 "A "dorsal organ" is early developed as a median thickening 

 of the ectoderm . . . . " — On the embryo in Mysidse (p. 179 — 80) : 

 "A pair of lateral thickenings of the epiblast appear \-ery earl)' 

 and, approaching each other on the dorsal side, fuse to form an 

 invaginated "dorsal organ"." But I may remark that Nusbaum 

 und W. Schreiber in 1898 (Biolog. Centralbl. Bd. 18, p. 742) 

 figured a transverse section of an embryo of My sis Lamorna' 

 showing the unpaired dorsal organ and a pair of dorso-lateral 

 organs, and all three organs have already begun to degenerate. 

 — At the Nebaliacea, Cumacea, Euphausiacea, Decapoda, and 

 Stomatopoda Caiman does not mention any dorsal organ. 



Giesbrecht writes (p. 156 — 57) that not only in Cladocera 

 but in "vielen anderen Crustacea ein anderes Organ von meist 

 driisiger Struktur, ebenfalls in der Dorsalwand des Kopfes, das 

 aber nur transitorisch (bei lunbryonen, seltener Larven) auftritt 

 und sich meistens schon friili zuriickbildet ; das ist die Nacken- 

 driise (Nackenorgan, Nackenschild, Dorsalorgan . . . .)". Among 

 the Malacostraca it is found early in the embryo in Arthro- 

 straca (consequently Tanaidacea, Isopoda, and Amphipoda), 

 in Cumacea, Mysidacea, perhaps also some Decapoda; "bei Lep- 

 tostraken scheint es eine unpaarige Leiste zu bilden"; finally 

 "als Nackendriise aufgefasst wird auch .... eine dorsale 

 Driisenscheibe in the Maxillargegend von jungen und adulten 

 ICuphausien etc.". I have been unable to find in the special 

 literature on the Euphausiacea any statement on the organ. 



In a small paper published in Jouni. Linn. Soc. Lond. vol. 

 XXIX (1903) the ]jre.sent writer ])ointed out an organ on the 



