382 MK. H. M. BEENAED ON THE 



lateral wall of the gut to near tlie diaphragm. They give off numerous branches which ramify among 

 the " peritoneal " cells (" fat body ") . This fact seems to connect them very closely with the digestive 

 functions. At the same time, it must not be forgotten that the mid-gut and the digestive tubules are all 

 provided with muscles, the waste of which would have to be got rid of. The tubules are lined with a 

 ciliated epithelium closely resembling that of the same tubes in Galeodes. 



In Thelyphonus, the tubes enter tlie mid-gut just posterior to the constriction in the 6tli segment, in 

 the region of the 7th segment, near the commencement of the enlargement of the gut which functions 

 as stercoral pocket, and which I believe to be the true hind-gut (PI. XXXIV. fig. 2). Arising at the 

 sides of the gut, they at first run backward, and twist about ventrally and laterally, closely applied ta 

 the wall of the gut. They then run sti'aight forward, and give oft' branches among the digestive tubules. 



In Phrijnus, the true chitin-lined hind-gut commences in the region of the 7th segment. I 

 have, however, been unable to ascertain the exact points of entrance of the Malpighian vesicles. They 

 can be seen twisting about the canal, as in Thdyjjhonus , close to the gut, and probably open in the 

 7th segment. 



The Malpighian vessels in the Spiders are well developed and ramify freely among the digestive 

 tubules within the "fat body," i. e. among the "peritoneal" cells. I have not myself been able to 

 ascertain their exact points of entrance. As in Galeodes, the tips of the tubules are often swollen 

 (Wasmann, Plateau, Schimkevitch). The tubules appear to be ciliated as in Galeodes, although it 

 is extremely difiicult to see the cilia. But the central core of faecal matter with which they are filled is 

 never in actual contact with the deeply-staining epithelial cells ; a clear sjjace, which, under high powers,, 

 looks like a band of cilia, separates the two. It is difiicult to see how the faecal bodies would find their 

 way along the tube, excepting by means of ciliary propulsion. Whether the tubes in the Spiders 

 have retained their original excretory function or not is difiicult to say ; they appear to have 

 undertaken another function — viz., that of assisting to remove the fsecal masses from the blind tips 

 of the alimentary diverticula (ii). 



The loss of Malpighian vessels as excretory organs in the abdomen may perhaps be correlated with 

 the absence of the coxal glands in the thorax in the majority of Spidei's. Compensation for the loss of 

 these two principal excretory organs has probably to be sought in the highly developed poison- and 

 spinning-glands, and perhaps in the fact that, in the Spiders apparently alone among Arachnids, 

 the skin of the abdomen is traversed by countless pores opening to the exterior. Excretory matter 

 might perhaps be discharged through these pores by the action of the hypodermal cells. 



The Pseudoscorpions (which have both spinuing-glands and cement-glands) appear to have no 

 Malpighian tubules. The part functioning as hind-gut commences in about the 2nd-;3rd abdominal 

 segment, and is coiled in the median plane (PI. XXXIV. fig. 3). 



The Phalangidse also appear to have no Malpighian vessels ; the hind-gut is large and lills up a great 

 part of the abdomen. In both these forms the digestive system shows no very complicated branchings, 

 and excretory matter might easily be removed by the blood circulating freely among it. 



In the Acari, the Malpighian vessels appear to be present in some forms and absent in others. 

 Michael found no traces of them in the Oribatidse, while Winkler has found them in the Gamasidae. 

 Wagner has found them in Ixodes, developed from the endoderm. 



There can be little doubt that the [Malpighian vessels are homologous structures throughout the 

 Arachnida. In Scorpio, Thelyphonus, and Phrynus, as we have seen, they open in the region of the 

 7th abdominal segment. The vexed question as to whether they belong to the mesenteron or proctodaeum 

 is finally decided by Galeodes, in which they obviously belong to the former. They are thus not 

 homologous with the Maljjighiau tubules of the Hexapoda. It is possible that they may be speciahzed 

 diverticula, homologous with those which anteriorly subserve digestion. In Scorpio they appear to 

 enter the gut dorso-laterally, as do the digestive diverticula. 



