T7PE PROTOCHORDATA. 



613 



the axes of the body ; they do not possess a metameric arrange- 

 ment, but are much more numerous than the muscle-plates of 

 the branchial region of the body, as many as one hundred 

 slits occurring in fully-developed individuals. In the parti- 

 tions between each pair of slits of 

 the same side of the body skeletal 

 rods occur which unite above with a 

 longitudinal bar lying above the 

 dorsal ends of the slits (Fig. 280). 

 A closer examination shows that 

 alternate bars differ in structure, a 

 condition due to the fact that each 

 pair of slits is primarily derived 

 from a single slit, which becomes 

 divided longitudinally by the growth 

 from its dorsal edge of a tonguelike 

 valve, which eventually fuses with 

 the ventral edge of the slit. The 

 condition of affairs then is almost 

 identical with what obtains in Ba- 

 lanoglossus, and, as in that form, the 

 skeletal bars are to be considered as 

 grouped together in threes, although 

 the two bars which in Balanoglossus occur in each tongue are 

 ftised together (tb) in Amphioxus, a continuity of the various 

 triads being thus produced. The similarity in the arrange- 

 ment of the branchial skeleton in the two forms may be seen 

 by comparing Fig. 280 with Fig. 276 (p. 605). 



It follows from this that the actual number of branchial slits in 

 Amphioxus is half the apparent number ; but even with this reduction the 

 metamerism of the slits does not agree with that of the muscle-plates. In 

 the early stages of development eight pairs of slits are developed which do 

 correspond metamerioally with the mesodermic somites, but later addi- 

 tional non-metameric slits are formed, and the original ones are pushed 

 forward at the same time, so that the metameric correspondence is lost. 



Blood-vessels occur both in the septa between the primi- 

 tive slits and in the tongues, and the edges of the slits are 

 ciliated, so that a current of water enters by the mouth, passes 

 through the slits into the atrial cavity, and thence to the 



Fig. 280. — Diagram of the 



Branchial Skeleton op 



Amphioxus (after Spengel). 



s = synapticula. 



sb = septal bar. 



tb = tongue bar. 



