xgoS] EXCRETORY ORGANS OF METAZOA. 587 



thelium ; they are not, as generally supposed, absent in the Aphid?e, 

 for Witlaczil (1882) has shown that the so-called pseudovitellus 

 represents them. In all other Insects these vessels are present, and 

 are usually delicate, cylindrical tubes, rarely varicose or ramose, 

 with their inner ends closed and the distal ends joining with the 

 intestine usually at the junction of the mid-gut and proctodseum, 

 and they may insert there singly or by one or several common ducts. 

 In some cases there are two different kinds in the same species. 

 Their number is often constant for a group as may be seen from 

 the following summaries taken from the observations of Dufour 

 (1833, 1841, 1851): in the Diptera there are usually four, rarely 

 five (Culc.v), and never more than four in the Hemiptera; there are 

 generally less than eight in the Coleoptera; six in Phryganids, 

 Termes, IMegaloptera (Corydalis, Sialis), Panorpa, eight in 

 Hcmerohia and Myrmeleo ; they are much more numerous in the 

 Orthoptera, Hymenoptera, Libellulidge and Ephemeridje.*' 



While Dufour called them " organes hepatiques ou biliaires,'' 

 subsequent work has proved conclusively that they are the main 

 excretory organs. 



According to the majority of investigators they arise as evagina- 

 tions of the ectoblastic proctodseum, and only in some Hymenoptera 

 do they first appear as ectoblastic evaginations at the posterior end 

 before the proctodseum forms. The largest number known in any 

 embryo is ten {Melanophis, Packard), which seems to be the single 

 case not in agreement with Wheeler's conclusion ('1893a) that no 

 more than six occur in embryos. \Mieeler concludes that six; is 

 the primitive number, while others have reasoned this to be four. 

 Only in the Termites are they more numerous in the larvse than; 

 in the adults. 



Homologies of tJic Malpighian J'csscls. — These have been com- 

 pared specially with the sericteries and trachese and more generallv 

 with nephridia of the annelidan type; and it is most convenient to 

 treat these relations at this place. Biitschli (1870) showed that 

 the sericteries and Malpighian vessels develop like the trachea, re- 



*A good review of their numerical and other relations is given by 

 Packard (1898). In the Thysanura (except Japyx) their number was found 

 by Grassi (1888) to vary from eight to sixteen. 



