THL; GASTllAIiA-THKOKY, ETC. 237 



system has developed itself, th(> physiological signiftcation of 

 Avhich to the animal organism in general is greater than that 

 of the younger blood-vascular system and the coeloni which 

 is connected with it. This opinion is confirmed by the 

 Plathelminthes, which still possess no coelom and blood- 

 system, but perhajis jjossess rudimentary kidneys (excretory 

 canals), and also by their universal occurrence throughout 

 the whole animal series, and, lastly, especially by the early 

 apj^earance of the " primary kidneys " in the embryo. All 

 which shows that we have here to do with a very old and 

 important arrangement of organization, which already existed 

 in the Acadomi before the formation of the blood- system and 

 the coelom, and has descended from thence to the higher 

 groups of animals. 



In the fifth line of succession the blood- vascular system 

 and the cojlom developed themselves first after the kidney 

 system. We have already shown that these two parts stand 

 in inseparable connection, and that the true body-cavity or 

 the coelom is to be considered as precisely the first commence- 

 ment of the vascular system. After the commencement of 

 the development of the intestinal fibrous layer, by its de- 

 tachment from the adherent dermal fibrous layer, a cavity is 

 first formed between these two muscular layers, Avhich fills 

 with the chyde which has transuded through the intestinal 

 wall. This was the coelom in its simplest form, and this 

 hsemochylic-systeni or primordial primitive blood-system has 

 subsequently become differentiated into two different systems 

 of fluids, into the lymph-system and the true blood-system.^ 



In the sixth line of succession the genital system has 

 first developed itself morphologically as an independent 

 system of organs (!) Certainly this has already been physio- 

 logically present the longest of all, before any other system 

 of organs became differentiated. We certainly already meet 

 with single cells scattered in the endoderm of the intestinal 

 tube in the sponges, some of which develop into germ- 

 cells, and others into sperm-cells ; and this was pro- 

 bably already the case in the Gastraeada. Only in all 



' A very dififerent view of the coelom and of the blood-system, as well as 

 of the kidney-system, has been developed by E. R. Lankester in his oft-quoted 

 article (' Annals and Magazine of Natural History,' May, 1873). He regards 

 these two systems of organs as identical, and thinks that the "excretory 

 organs or water-vessels " of the Acoelomi form the first commencement of a 

 body-cavity, and that this coelom is therefore opened externally from the 

 beginning. On the contrary, my opinion is that the coelom is prin)arily 

 closed, and originated subsequently to and independently of tlie older 

 primary kidney-system. The connection of the two would then be secondary. 

 Tlie ontogenesis of the Bilateria seems to me to contradict E. 11. Lankester's 

 opinion. 



