28 



CCELOMIC SACS OF MOLLUSC A 



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

 the animal series is a difficult one and full of pitrfalls for the 

 speculative morphologist. 



By the establishment of the existence of the coelom in an 

 independent condition in Mollusca and Arthropoda, having so far 

 as embryological observations have gone, an enterocoelous origin 

 (von Erlanger in Paludina, 1 Sedgwick in Peripatus), and by the 

 recognition of the spaces at one time confounded with coelom in 

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



-c. 



Pio. 15. YOUNO EMBRYOS OF THE GASTROPOD MOLLUSC BITHYNIA TEXTACCLATA TO SHOW 



THE APPEARANCE OF THE CCELOM AT AN EAKLY PERIOD AS A PAIR OF POUCHES 

 DERIVED FROM THE WALL OF THE ARCHENTERON (? ES Solid Or hollOW OUtgPOWths). 



A, frontal section ; B and C from the right side. o, region of the anus ; U, blasto- 

 pore; c, coelom; mes, epithelial cell -wall of the coelom; ent, endoderm ; m, mouth; 

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

 (After von Erlanger, from Korschelt and Heider.) 



haemoccel, the theory of the coelom 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 coelom 

 throughout the animal series above the Protozoa and Enterocoela, 

 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- 



1 See Fig. 15 and the explanation. 



