26 THE VOYAGE OF H.M.S. CHALLENGER. 



believes he has observed segmental organs in each of the seven body segments. Whether 

 M. Huet be right in considering these organs "organes glandulaires . . . qui s'ouvrent h, 

 la partie superieure des ^pimeres, de chaque cote, par une ouverture en crible " as segmental 

 organs, I will not discuss. To judge from his description they have not the typical 

 structure of true segmental organs which are to form an open communication between the 

 body-cavity and the exterior. 



Two other sets of glands of Arthropoda, and more especially of Crustaceans, are 

 perhaps more nearly related to the segmental organs ; they are the antennal glands of 

 the larvae of many Entomostracans and of the full-grown Malaeostracans ; and the shell- 

 glands of full-grown Copepoda and Phyllopoda. According to Grobben^ they have 

 nearly the same structure and must be regarded as homologous organs (homodynamous 

 they are called, more accurately I think, by R. and 0. Hertwig ^) ; both are com- 

 posed of a little terminal sack (Endsackchen), and a channel (Harnkanalchen) which 

 opens at the surface of the body. Moreover, the cells covering the interior of the little 

 sack in the antennal and shell-glands show a complete resemblance. An open communi- 

 cation with the body-cavity has, however, never been observed in the case of these 

 organs ^ ; if they really are to be compared with segmental organs, there can be no 

 question that they have degenerated from their original condition. 



Should there ever be discovered an intermediate form between a true segmental or'gan 

 such as that of Scalpellum and a shell-gland as observed in the Copepoda, then in the 

 first place the homology of the apparatus may be accepted ; but in the second place it 

 will then also be possible to give a more solid basis for demonstrating the homologies of 

 the extremities of Cirripedia and Copepoda than has been the case hitherto. When treat- 

 ing of the female genital apparatus and its orifice at the base of the first cirrus I hope to 

 point out that there is sufficient reason for admitting that a second pair of segmental 

 organs, though in a slightly modified condition, is present in the Cirripedia also. 



Finally, I will not take leave of this subject without stating as my opinion that the 

 segmental organ which I have described is physiologically an organ of an excretory 

 nature The condition of the material at my disposal did not allow of my attempting a 

 chemical investigation of the contents of the cells, and so it is only from analogy that 

 this conclusion has been arrived at. It is fairly supported, I think, by the presence of 

 muscle-fibres with numerous cavities between them, such as have also been observed 

 by Grobben (loc. cit., p. 105) in the neighbourhood of the antennal glands of the 

 Decapoda. 



' C. Grobben, Die Antennendriise der Crnstaceen, Arb. Zool. Inst. Wien. Bd. iii. 1880. 



2 E. and O. Hertwig, Die Ccelomtheorie, Jenaische Zeitschr. Bd. xv. pp. 1-150, 1882. 



' According to Sedgwick {Quart. Journ. Micr. Sd., vol. xxiv. N.S., pp. 46, 47, 1884), the nephridia of the Inverte- 

 brata are developed from solid masses of cells derived from the wall of the ccelom ; a communication with the body- 

 cavity in that case would represent a secondary stage. 



