I90S 



EXCRETORY ORGANS OF METAZOA. 589 



conclude, with Palmen, that while these may all have had an essen- 

 tially similar beginning no one of them has been derived from the 

 others. The Malpighian vessels may well have been hypodermal 

 glands that have invaginated with the proctodaeum, and for this 

 speaks their independent origin in the embryos of some Hymen- 

 optera. In this connection it is interesting to note the conditions 

 in the larv^ of Phryganids, as described by Henseval ( 1896) : here 

 there are three pairs of ventro-median glands (glands of Gilson) ; 

 and Henseval regards the Malpighian vessels as homologous glands 

 of the last segment, and the proctodaeum as their unpaired portion 

 that has secondarily joined with the mid-gut. If we omit this ex- 

 planation of the proctodaeum as being problematical, the comparison 

 of -Malpighian vessels with segmental glands placed anteriorly on 

 the hypodermis might well hold.'^ 



Homologiies of XcpJiridia. — Here there are in the first instance 

 the genital ducts, that develop as coelomic evaginations (Wheeler, 

 1893, Nassonow, 1886) ; Wheeler has shown that all the abdominal 

 coelomic sacs develop such peritoneal funnels, but that only those 

 of one particular somite reach the exterior and become functional 

 genital ducts. He also (18930) holds that the oenocytes represent 

 ectoblastic remains of nephridia. The prothoracic gland of 

 Dicranura has been considered homologous (Latter, 1897). Nasso- 

 now (1886) has concluded a like relation for the head glands of 

 Caiiipodea, all salivary glands, the maxillary glands of Lepisma, and 

 the extensible vesicles of the Thysanura; but Oudemans (1887) and 

 Haase (1889) combat this view and regard the extensible glands 

 at least as not nephridial but as respiratory skin glands. Wheeler 

 (1893a) considers the fat-body to represent mesoblastic remains of 

 nephridia; some of its cells are proved to be excretory (Wheeler, 

 Cuenot, 1895, Bruntz, 1903), and Anglas (1901) suggests that 

 such cells compose an " accumulating kidney " that functions during 

 the substitution of Malpighian vessels in the metamorphosis. 



Nephrocytcs. — According to Bruntz (1903) these cells are 

 labial in Machilis, and in it as in Lepisma are found also on the 

 fat-body ; in larval Neuroptera on the wing muscles ; in Ephemera 



' Other ectoblastic glands regarded as excretory are the segmental globi- 

 form glands of Ocypus (Georgevitch, 1898). 



