28 



CCELOMIC SACS OF MOLLUSCA 



of tracing the origin and history of the blood-vascular system in 

 the animal series is a difficult one and full of pit-falls for the 

 speculative morphologist. 



By the establishment of the existence of the ccelom in an 

 independent condition in Mollusca and Arthropoda, having so far 

 as embryological observations have gone, 'an enterocoelous origin 

 (von Erlanger in Paludina/ Sedgwick in Peripatus), and by the 

 recognition of the spaces at one time confounded with ccelom in 

 those great phyla, as being in reality swollen blood-vessels or 



a. 



■mi^ 



Fio. 15.— YouNO Embryos of the Gastropod Mollusc Bithynia tentaculata to show 



THE ArPEARANCE OF THE CCELOM AT AN EARLY PERIOD AS A PaIR OF POUCHES 

 DERIVED FROM THE WaLL OF THE AlfCHENTERON ('? aS Solid OF lloUOW CUtgrOWths). 



A, frontal section ; B and C from tlie right side. a, region of the anus ; hi, blasto- 

 pore ; c, co-lorn; in.es, epithelial cell -wall of the ooeloni ; tnt, endoderm ; to, mouth; 

 sd, shell-gland ; t, prostomial region ; v, cells of the ciliated band of the velum. 

 (After von Erlanger, from Korschelt and Heider.) 



haemoca'l, the theory of the crelom is brought to a second stage. 

 The results thus emphasised have been gained during the fifteen 

 years which succeeded the publication in 1881 of the Hertwigs' 

 Coelomtheorie. The existence and the unity of the co'lom 

 throughout the animal series above the Protozoa and Enterocwla, 

 its derivation in all cases from pouch-like growths of the archen- 

 teron either actually or with delayed appearance of the lumen, as 

 suggested by me in 1875, seems now to be established with some- 



' See Fig. 15 and the explanation. 



