i86 



COMPARATIVE ANATOMY 



the ventral vessel and backwards in the dorsal vessel. This is' 

 characteristic of all vertebrates, and is exactly the reverse of 

 what happens in worms and Arthropods. 



The excretory system of Amphioxus consists of a series of 

 short ciliated excretory tubes situated near the upper end of 

 every tongue-bar in the pharyngeal region. Each tubule com- 

 municates on the one hand with the dorsal coelom by several 

 ciliated openings or funnels, and on the other hand with the 



Fig. 46 



Three excretory tubules with the neighbouring portion of the pharyngeal waU, after 

 Boveri. The so-called "thread-cells" are only shown in one of the tubules. at,Of 

 atrial opening of an excretory tubule *, Co, cffilomic openings ; ca^, dorsal ccelom passing 

 into a coslomic canal in a primary bar ; et, excretory tubule with "thread-cells " ; Id, 

 ligamentum denticulatum ", sy, cross-bar or synapliculum ; P,P, primary gill-bars; 

 T,T, tongue-bars. 



atrial cavity by a single opening. Three tubules are shown 

 in their natural position in fig. 46. Each is a short curved 

 tube bent nearly at a right angle, the anterior limb of the 

 tube being vertical, the posterior Hmb horizontal. Not only 

 does the tube open by both ends into the coelom, but between 

 these openings is a variable number — usually two or three, 



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