196 



ZOOLOGY, 



not open into the ccelom they are communications between 



the cavity of the alimentary canal (pharynx) and the ex- 

 terior. This apparent 

 body-cavity, then, in 

 Amphioxus is not the 

 ccelom, but something 

 new, not found in the 

 dogfish : it is called the 

 atrium. The atriopore 

 is its opening to the 

 exterior. Behind the 

 atriopore there is a 

 blind extension of the 

 atrium on the left side 

 only. 



The outer surface of 

 the pharynx must cor- 

 respond to the outer 

 surface of the dogfish's 

 body where the gill-slits 

 open. If the student 

 will imagine that in 

 the dogfish a pair of 

 longitudinal ridges of 

 skin and muscle were 



formed just above the gill-slits, and that from these there 



grew down flaps which met in the ventral middle line so 



as to hide the external 



openings of the gill-slits, 



he will get a rough notion 



of the formation of the 



atrium in Amphioxus. (See 



also fig. 116.) 



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f*G/OAf ACG/Off 



4. The Ccelom. The ^ 106 ._ DlAQRAM OF INTEBNAL OABT OF 



true CCelom may be easily PART OF CCELOM OF AMPHIOXUB. 



found in a transverse 



section taken behind the atriopore (fig. 105). It occupies 

 its usual position, but owing to the intestine being quite 

 free from any coiling, the ccelom is a very narrow space. 



Fig. 105. TRANSVERSE SECTION OF AMPHIOXUS 

 IN INTESTINAL REGION. 



The backward extension of the atrium on 

 the left side is not shown. 



