220 E. A. ANDREWS. 



If we now trace the dorsal fin anteriorly in the region of the 

 posterior myotomes we find that the lymph canals that penetrate 

 its substance become arranged in definite, parallel lines radiating 

 upward from near the neural cord. Several of these canals occur 

 opposite each myotome and some connect with large lymph spaces 

 now appearing just above the neural cord. As seen in Fig. 25, a 

 surface view from an animal living in sand with Bismarck brown, 

 granules of brown substance occur in large cells lining the canals 

 and the lymph cavities. These cavities become larger and more 

 regular anteriorly and form the fin-ray spaces of the dorsal fin, 

 three or four for each myotome. Henceforth the dorsal fin becomes 

 more and more predominantly made up of these lymph spaces which 

 succeed one another in a constant crowded series as seen in Figs. 1, 

 2, 3, 4 and in Fig. 17. 



The lymph canals still remain and at places open into the fin- 

 ray spaces, from the summits of which they extend out into the fin. 

 Though at first nearly vertical (Fig. 25) these canals bend backwards 

 more and more until, anteriorly, they come to run nearly parallel 

 to the dorsal edge of the fin and will appear in transverse sections 

 as in Fig. 22. 



The fin-ray spaces are lined by a membrane with distinct cells 

 projecting into the lumen (Fig. 17). Though at first quite near the 

 nerve cord the spaces gradually rise, anteriorly, as the muscles 

 become vertically deeper, and thus come to be separated from the 

 nerve cord by a considerable space as seen in Figs. 22, 14, 12. 



Projecting into each well formed fin-ray space, but not into the 

 imperfect posterior ones represented in Fig. 25, there is a ventral 

 mass of solid connective tissue, the so-called fin-ray. These fin- 

 rays (Fig. 17) vary much in size and shape in differently prepared 

 specimens but never appear as prominent as in some species of 

 Amphioxus. They are not paired but single, median upgrowths 

 from the ventral wall of each fin-ray space. 



About the middle of the branchial region the proportions of the 

 fin-rays and fin-ray spaces are as in Fig. 17, but posteriorly they 

 became more crowded and deeper vertically, most so over the region 

 of the atriopore and just posterior to that region. Anteriorly the 

 spaces become more shallow and longer till finally the fin-rays 

 cease to exist over the pre-oral chamber while the fin-ray spaces 



