lo PHLEBCEDESIS 



and of certain excretory organs only.^ There is reason to believe 

 that this small size of the ccelom in the Arthropoda is not due. to 

 a retention of the original small size of the cwlomic sacs, but is 

 to be ascribed to a swelling of another and independent liquid- 

 holding cavity, namely, the blood-vascular or haemal system which 

 has filled up the space formerly occupied by a capacious calom. 



(6) The theory of Phlebcedesis — the Ccelom and the Hcemoccel. 



This swelling of the peripheral portions of the hnemal system 

 may be called Phlebcedesis, and the lacunar blood-holding spaces 

 resulting from it form a " Haemocoel " which has no connection 

 with the ccelom, but has to a large extent encroached on the space 

 which once was occupied by ccelom and caused the reduction of 

 that organ to perigonadial and epinephric remnants. 



In the MoUusca the ccelom also appears to have undergone re- 

 duction in volume. The pericardial cavity and the more or less 

 extensive ramifications connected with it, as well as the gonadial 

 sacs, are the ccelom of Molluscs. Until recently (1885) it was 

 erroneously supposed that the pericardial system of the Mollusca 

 contained blood. It does not ; it is, on the contrary, entirely dis- 

 tinct from the blood-system. In the more primitive Molluscs 

 (some Neomeniffi and Cej^halopoda) the pericardial and perigonadial 

 sections of the coelom are in continuity, and in them also the 

 blood-system appears more completely developed in the form of 

 cylindrical tubes or " vessels " than in other Molluscs. But in all 

 Molluscs as in all Arthropoda - the process of Phlebredesis has 

 taken place, and a voluminous, irregularly distended system of 

 blood - spaces — a Ha^mocciel — has suppressed and replaced to a 

 large extent the ccelom. In Lamellibranchs the paired, widely 

 ramifying tubes of the organ of Keber, leading out of the peri- 

 cardial cic'lom, appear to be the reduced representatives of a 

 formerly voluminous ccelom. 



It appears that neither in Arthropoda nor in Mollusca is 

 there any breaking through of the swollen blood-cavities into the 

 coelom. 



Before the theory of Phlebcedesis was established, it was 

 supposed l)y many zoologists (of whom I was one) that the ca'lom 

 and bipod-system were of one common origin, and that in Mollusca 

 and Arthropoda they were in open contiiuiity, and, in fact, to a 

 large extent undifferentiated. This has now been shown to be an 

 erroneous view : the ctclom is distinct from the vascular system in 



^ Possibly other reinnants of the co-lom exist as spaces in connective tissue. 

 * It remains to be ascertained wliether the Copepod Crustacean Leriiauthropus 

 with its tubular vascular system containing red blood is an exception or not. 



