Morphology of Organs. 157 



kidney of various vertebrated animals each separate tube, 

 which development proves to be a nephridium, ends in a 

 dilated sac, into which is thrust a coil of blood-vessels, the 

 so-called glomerulus. This dilated sac, which is lined by an 

 epithelium more flattened than that which lines the excretory 

 tubules themselves, has been shown on very good grounds to 

 be a cut-oflf bit of the ccelom itself. So that, in this case, we 

 have an imdoubted nephridium opening into a small sac, which 

 is coelomic, a state of affairs which we have ex hypothesisi in 

 the crayfish. This, however, is rather an argument from 

 analogy ; it shows that it is, at any rate, reasonable to regard 

 the terminal sac of the crayfish's excretory organ as a fragment 

 of ccelom. There are stronger arguments still. Peripatus has 

 been shown to be a very archaic form of arthropod animal. 

 Among other characters which it possesses, which show affinities 

 to the segmented worms, are a regular series of undoubted 

 nephridia. These nephridia do not open into a wide space, 

 like the ccelom of Lumbricus, but into terminal vesicles, which 

 have been proved developmentally to be coelomic pouches; 

 these, though larger, are exceedingly like the terminal sac of 

 the green gland of Astacus. Finally, in Peripatus, each 

 nephridium opens on to the base of a limb as does' the green 

 gland of Astacus. The limbs correspond, and it seems likely, 

 therefore, that the corresponding position of the nephridiopores 

 has significance. In the event of its being shown that the 

 terminal sac of the green gland of Astacus is not a coelomic 

 sac, there is still no decisive reason for removing the green 

 glands from the category of nephridia ; for in that case there 

 will be no ccelom at all in the neighbourhood of the green 

 gland, and if they are homologous with nephridia they cannot 

 have an internal opening into the ccelom, for the very good 

 reason that there is no ccelom to open into ! 



In the same way, from a comparison with other animals, 

 some arguments can be put forward to show that the 

 Malpighian tubes of the cockroach are referable to the same 

 category of organs. At first sight they differ widely, for they 

 are apparently appendages of the gut ending blindly, and com- 

 parable to the hepatic appendages, which no one has attempted 



