ARTHROSTRACA AND CUMACEA. 151 



A dorsal organ developed in this Avay has been proved by F. Muller to exist 

 in many Araphipoda ; also in some Isopoda (Fig. 76) — Cymothoa, Claus and. 

 Bullae, No. 81 ; Praniza, Dohrn. An interesting variation is found among 

 the Amphipoda in the genus Orchestia, where the dorsal organ originally 

 develops on the lateral edge of the germ-band, and only later shifts towards the 

 median dorsal line (Uljanin, No. 75 ; Rossijskaya, No. 72). 



Another method of development of the dorsal organ is seen in Onisais 

 (Dohrn, No. 83 ; Bobretzky, No. 80), where the cells of a large area thicken 

 to form a saddle-shaped plate adhering closely to the larval integument (Fig. 71 

 A, do, p. 140). This plate, in the further course of development, becomes more 

 and more separated, by an infolding of its margin, from the embryo, with which 

 finally it is connected only by a thin, hollow column. The plate, when this 

 connection is lost, undergoes disintegration. A similar dorsal organ is found 

 in Ligia (Fr. Muller, No. 4 ; Rosalie Nusbaum, see J. Nusbaum, No. 39, 

 p. 168). 



A dorsal organ similar to that in the Amphipoda has been observed in the 

 Cumacea (Dohrn, No. 36). In Mysis it is originally paired (Fig. 77, d). The 

 paired disc-like thickening found in Tanais (Fr. Muller, No. 4) connected 

 with the larval integument, might also be thus interpreted. Among the 

 Decapoda, the dorsal organ is found in a reduced condition in Crangon. 

 (Kingsley, No. 53), and perhaps (?) also in Pandalus and Palinurus (Dohrn, 

 No. 45), as well as in Homarus (Herrick, No. 50). 



Among the various explanations given of this organ, the one originated by 

 Fr. Muller (No. 4), and put forward later in more detail by Grobben (No. 11), 

 has met with the widest acceptance. According to these authors, the dorsal 

 organ is the vestige of a once functional adhesive structure, equivalent to the 

 nuchal gland {neck-gland, Fig. 72 C, n), which is still functional in the young 

 stages of the Phyllopoda and is there sometimes retained throughout life. Such 

 a nuchal gland was also found by Grobben and Urbanowicz in Copepoda 

 (Cyclops, Ergasilus), and by Grobben in Euphausia. Although this homology 

 appears very probable, we do not think it entirely established. It is possible, as 

 already mentioned, that the dorsal organ is merely the expression of the com- 

 plication of the blastodermic covering of the food-yolk caused by the lateral 

 growth of the germ-band. This complication would then be expressed by an 

 invagination in the Amphipodan type, and by a nipping off in the Oniscus type. 

 These processes are perhaps analogous to the formation of the dorsal organ in 

 the Insecta. 



Among those structures which have often been homologised with the dorsal 

 organ, are the paired lobe-shaped appendages (Fig. 74, I) of the Asellus embryo. 

 The true significance of these latter was first explained by Claus (No. 82) by a 

 comparison with his observations on the young stages of Apseudes. In Apseudes, 

 in the maxillary region, there are two wing-like integumental folds ; these are 

 the rudiments of a shell which spreads over the small respiratory cavity, and 

 under which the palp of the anterior maxilla and the vibratile epipodial lamella 

 (scaphognathite) of the maxillipede come to lie. In Asellus the shell-fold is 

 reduced to a triangular rudiment which (Rathke, No. 88) may function as an 

 embryonic gill. 



The Anisopoda just mentioned (Tanais, Apseudes), in the conformation of their 

 embryos, most resemble the Isopoda, having in common with them the dorsal 

 curvature and the absence of the seventh pair of thoracic limbs. In other 

 respects, they more nearly approach the Cumacea, all the abdominal limbs except 



