THE LOWER VBRTBBRATA. 197 



Traced forward, it is found not to stop short, as in all 

 our previous types, but to extend right forward to the 

 mouth-region. But obviously it cannot form a con- 

 tinuous longitudinal space where there are gill slits, and 

 so we find that it is only above and below the gill-slits 

 that longitudinally-continuous coelomic spaces can exist. 

 These are the paired dorsal and median ventral coelomic 

 canals, which are joined by lateral branchial canals in 

 the gill-arches. These conditions may be best understood 

 from fig. 106, which represents an internal cast of the 

 ccelom, such as we might get by filling it with some solid 

 material and then destroying all the soft parts around. 

 (See also thecoelom in figs. 104 and 107.) Thus instead of 

 a continuous cceloniic space we have a series of ccelomic 

 channels. These cannot serve the function we have 

 attributed to the ccelom hitherto : what other function 

 they may have we may see presently. 



5. Mouth and Pharynx. The alimentary canal is a 

 straight tube, divided into two portions about equal in 

 length a respiratory portion or pharynx, and a digestive 

 portion or intestine. The mouth is merely a round hole in 

 the anterior end- wall (velum) of the pharynx, and is 

 surrounded by a sphincter-muscle, from which twelve 

 little velar tentacles (not to be confused with the prae-oral 

 cirri) project inwards. The pharynx is high and narrow ; 

 its walls are perforated by an enormous number of long and 

 narrow gill-slits with narrow gill-arches between them. They 

 are also divided by horizontal bars which run across from 

 one arch to the next, so that the whole side-walls of the 

 pharynx (which are very thin) become little more than a 

 trellis-work or sieve. We can now understand the ad- 

 vantage of the atrium : it enables an exceedingly delicate 

 sieve-like pharynx to co-exist in the same region of the 

 body with fully developed swimming muscles. In the dog- 

 fish there is no need of an atrium, because the pharynx is 

 relatively short, and there is plenty of room for muscles in 

 its floor and between the gill-slits. 



6, The gill-slits of Amphioxus are exceedingly numerous, 



