Phylogeny of the Arachnida. 295 



Ixodes)^ so that, unless sufficiently early successive stages are 

 studied, they may be regarded as outgrowths from the rectal 

 vesicle itself. From this point of view the lower half of the 

 8 in Schimkewitsch's figure represents a portion of the 

 mesenteron ; Balfour and Morin did not observe sufficiently 

 early stages*, as can be proved from the fact that they did 

 not notice tlie formation of the posterior process of tlie 

 mesenteron, which is distinctly visible in Morin's fig. 30 (in 

 Theridion rnaculatum). A necessary stage has also been 

 missed by Kischinouye, since he has not observed the separa- 

 tion of the rectal vesicle from the anal invagination, which in 

 all probability takes place before the union of the Malpighian 

 tubes with the cloacal expansion [i. e. rectal vesicle), as is 

 likewise the case in Ixodes ; it is probably this circumstance 

 that has also induced the investigator in question to ascribe 

 to the vesicle an independent origin (from the mesoderm). 



The difference in the development of the Malpighian tubes 

 is also indicated by their relation to the alimentary canal in 

 the adult Spiders : in some they open into the cloacal expan- 

 sion (Bertkau, No. 9, pp. 415-416), in others into the 

 posterior process of the mesenteron (Loman, No. 41). Owing 

 to the place at which they open into the intestine (in Cteniza, 

 and also in Epeira^ Tegenaria, and Mygale), and in conse- 

 quence of their structure, Loman (No. 41) regards the 

 Malpighian tubes as being derived from the mesenteron. 

 Without attributing great importance to the histological 

 structure of the Malpighian tubes for deciding as to their 

 origin, I nevertheless consider it necessary to point out that 

 in the adult male of Ixodes calcaratiis, Bir., and in certain 

 larva3 (No. 66, figs. 82 and 83), the cells of the intestinal 

 lobes and of the central stomach portion contained granules 

 of an intense light blue or blue pigment, while this was not 

 the case in the Malpighian tubes. 



I therefore consider that, judging from the development of 

 the Malpighian tubes in the Scorpion, in Ixodes, and Agelena, 

 from their relation to the alimentary canal in the adult Scor- 

 pion and in certain Spiders, and, lastly, from their structure 

 in adult Spiders, the so-called Maljnghian tubes in all Arachnids 

 are develoi)ed jrom the endoderm. 



The second resemblance to the Malpighian tubes of the 

 Insects (and Myriopods) is a functional one. Yet it will 

 probably appear on closer investigation that this similarity is 

 by no means so essential as it seems ; we are already aware 

 of important differences between these structures in the 



* The stages whicli they studied were probably those immediately 

 foUowing the one shown by Morin in his fig. 29. 



