158 Elementary Zoology. 



to prove to be excretory organs comparable to nephridia. In 

 the first place, however, the Malpighian tubes are appendages 

 of the end gut, which is proctodseum, and therefore of epiblastic 

 origin ; morphologically, in fact, they open on to the exterior 

 of the body, just as do the archinephric ducts of vertebrates, 

 which debouch into the cloaca. As to the absence of any 

 communication with the ccelom, there is no coelom that has 

 been described for them to open into. And since in certain 

 worms, where there is a copious coelom, the internal aperture 

 of the nephridia is still sometimes obliterated (C^i^ATgarf^-), it is 

 not difficult to understand that this portion of the nephridium 

 may have disappeared in Periplaneta. Moreover, in certain 

 worms (the earthworm Acanthodrilus, certain Gephyreans) there 

 are undoubted nephridia which do open into the cloaca, apart 

 altogether from the vertebrates, which are probably further 

 away from the tracheata. There is thus no insuperable 

 objection to regarding the Malpighian tubes as modified 

 nephridia. This, however, is not the same thing as saying 

 definitely that they are referable to the same category. 



Gonads and Genital Ducts. 



In most Coelomata genital ducts or gonad ducts exist. In 

 some of the more lowly organized forms the genital products, 

 when ripe, escape by the mere rupture of the body-wall ; such 

 is the case, for instance, in certain simply organized worms, 

 such as Polygorditis. In other animals the genital products 

 escape through definite pores, which place the ccelom in 

 which they are formed in communication with the exterior. 

 The cyclostomatous fishes are an instance to the point; so, 

 too, certain lowly organized worms, such as ^olosoma. It 

 seems, however, to be not settled in these cases whether the 

 pores are not the last remains of definite ducts. They certainly 

 appear to be so in such animals as the worm EnchyfrcRus. 



In those animals which have separate genital ducts there 

 are anatomically two kinds, which seem to be more distinct 

 than they are. In such cases as the oviducts of the frog and 



