227 



of organisms at whatever part of the animal is most suitable for inter- 

 change of the gases, so that the anatomical comparison of respiratory 

 organs shows a very diverse and heterologous set of ectodermal and 

 endodermal organs which, morphologically, have rarely any connection. 

 In the case of »Nitrogenous« waste products a somewhat different 

 state of affairs holds. These poisonous products must be got rid of as 

 speedily as possible, so that excretory organs will tend to be developed 

 at any point of the ectoderm, at which a current leaves the organism. 

 The chief points fulfilling this requirement are the monocytic egestive 



Fig. 7. 

 Fig. 5. î'ig- 6- 



\1' 

 • Fig. 5. This and the figures (8 — 10) represent the relationship of the organs 

 of excretion and monocytic egestion, shown in phylogeny of nephridia. The com- 

 plications in connexion with the sexual function have been ignored. 



A coelomic pore for monocytic egestion (primary function). (Hemichorda and 

 Cephalochorda.) 



Fig. 6. Commencement of location of excretory cells on walls of ectodermal 

 invagination. 



Fig. 7. Increase of excretory surface by coiling as found in typical nephridium. 



aperture (nephrostome) and the polycytic anus (Fig. 4) so that around 

 each nephrostome and in the proctodaeal area nitrogenous excretory 

 organs usually occur in the former case forming the main part of the 

 nephridium, arising ontogenetically from the ectoderm, and con- 

 sisting of active cells discharging their nitrogenous products into the 

 nephrostomial duct, and in the latter case in the Malpighian tubules, 

 arising ectodermally ^^ and falling into the proctodaeum having preci- 

 sely the same relationship to the polycytic anus as the nephridial cells 

 to the nephrostomes (Figs. 5 to 10). 



The same principles are involved in the excretory system of 

 Amphioxus, where excretory cells ''^ (ectodermal) are said to line the 

 atrial cavity, where, of course, there is a current to the exterior not 

 only from the nephrostomial openings, but from the branchial aper- 

 tures. 



6* »The Malpighian bodies arise as two pairs of outgrowths the epiblast of the 

 proctodaeum.« F. M. Balfour, Comp. Emb. Vol. I. p. 414. 



65 P. Langerhans, Arch, für mikroskop. Anat. XII. 1876. 



14* 



