32 



ANATOMY OF AMPHIOXUS. 



Besides the rods in the gill-bars, there is a series of 

 paired skeletal plates lying immediately below the endo- 

 stylar epithelium (Fig, 13). These plates correspond in 

 number to the primary gill-slits. Their shape and arrange- 

 ment are shown in Fig. 14. They slightly overlap each 



other, and alternate 



P^^ 



with one another just 

 as the primary gill-slits 

 alternate. This alter- 

 nation of paired struc- 

 tures is of very general 

 occurrence in Amphi- 

 oxus, and affects almost 

 every system of organs, 

 — such as muscular, 



Fig. 14. — Lower portions of skeletal rods of nerVOUS rCDroduCtive 

 pharynx with three pairs of endostylar plates, ' 



seen from above. (After spengel.) and branchial systcms. 



The substance of the skeletal rods passes into y , f *- j 



that of the endostylar plates (f./), thus producing ^^ maybe Stated aS a 



an arcade like the cover of a shoe (Spengel). o-gncral rule tO which 

 .y. Cross-bars (synapticula). 



there are some excep- 

 tions, that with regard to the paired organs of Amphioxus, 

 the organs of one side (e.g. myotomes, primary gill-slits, 

 gonads, spinal nerves) do not lie opposite to their autv)icrts 

 on the other side, but alternate with them. 



Branchial Bars. 



The structure of the branchial bars is shown in section 

 in Fig. 15. Both kinds of bars, primary and secondary, 

 have the same general appearance, being compressed and 

 band-like, but the secondary bar is the smaller of the two. 



The chief point of difference between them is, that in 

 the primary bar a portion of the coelom is involved, which 

 is absent in the secondary bar. In the case of the primary 



