i9o8.] EXCRETORY ORGANS OF METAZOA. 583 



in part mesoblastic. All those of the first kind may well be homo- 

 logous, but those of the second kind are more probably hetero- 

 geneous structures. 



Other Excretory Organs. — According to Cuenot (1899) the fol- 

 lowing structures are excretory : in the Amphineura and Scaphopoda 

 connective tissue cells ; in prosobranch and opisthobranch Gas- 

 teropoda similar cells as well as cells of the liver ; in the Pelecypoda 

 pericardial glands ; and in the Cephalopoda phagocytes and the 

 gill-hearts. 



24. Tardigrada (Arctiscoidea.) 



A pair of glands opening into the rectum were supposed by 

 Plate (1888) to be excretory, and he compared them with the 

 Malpighian vessels of the Acarina. But neither he nor Basse 

 (1905), who has furnished a fuller description, were able to find 

 excretory products in these organs. Nothing is known of their 

 development. 



25. Pycnogontda (Pantopoda). 



Dohrn (1881) has described problematical " Excretionsorgane "' 

 within the cavity (blastocoel) of the fourth or fifth joint of the 

 second extremity, or the third or fourth joint of the third; each 

 has an external opening placed upon a small tubercle; in genera 

 where the named extremities are absent, these organs are found 

 in the wall of the body at points opposite the missing extremities. 

 These organs lie in extremities that lack reproductive organs, and 

 for that reason Dohrn suggested they may have some homodynamic 

 relation to the latter. 



Kowalevsky (1892) fovmd by injections of acid fuchsine that 

 the stain is taken up by small hypodermal glands placed in Phoxi- 

 cliilns on the borders of the three anterior segments and on the bases 

 of extremities fourth to seventh, and in Pallene and Aminothea in 

 the lateral processes of trunk segments and in the first joints of the 

 extremities. 



26. Crustacea. 



Shell Glands {Maxillary Glands). — These have been described 

 for the Phyllopoda (Leydig,- i860, Weismann, 1874, Claus, 1875, 

 Dohrn, 1870, Nowikoff, 1905), Copepoda (Claus, 1877, Nettovich, 



