138 



BIOLOGICAL LECTURES. 



Six gills on both sides, 6 on one, 7 on the other side, and 

 7 gills on both sides. This series is from the Cape of Good 

 Hope, /. c, practically Indian Ocean, and was discovered by 



Johannes Miiller in 1834. Aluller originally 

 gave a separate specific name to each of these 

 varieties but later concluded that they formed 

 only a single species. No representatives of 

 the series with more than 7 gills on both sides 

 and less than 10 on both sides, have as yet 

 been recorded, or so far as I knov/, observed. 

 This leaves a gap of 2 gills. But from 10 

 gills on both sides, up to 14 gills on both 

 sides the series is complete and is as follows: 



10 gills on both sides, 1 1 gills on both sides, 



1 1 gills on one side, 12 on the other, 12 gills 

 on both sides, 1 2 on one side, 1 3 on the other, 

 13 gills on both sides and 14 gills on both 

 sides. It is interesting to note the fact 

 as first made out by Willey that during 

 its larval development, the number of 

 primary gills laid down in Branchiostoma ^ 

 varies from about 8 to 16, with an average 

 number of 14. This may be a mere coin- 

 cidence, but I am disposed to look upon 



partly exploded and un- ^|^jg ^^ ^ f,^^,}- ,)f fundamental importance. 



ravelled thread-cell. </, -^ , , r ^ ^ i- i ^.i. „ ^\ ^c 



c •, , • 1 Kowalewsky first studied the growth of 



a portion of a coil which -' ^ 



has slipped off the cell, the gills in Branchiostoma, and Willey has 

 A the unravelled thread, gi-eatly increased our knowledge of the 

 much magnified. proccsscs of differentiation met with in 



these organs. Unlike other members of the vertebrate stock 

 this animal has the gills on one side laid ilown before those of 

 the opposite side make their appearance, but this is a mere 

 retardation and not an essential difference. The second gill- 

 slit is the first to appear and is soon followed by the first, after 

 which increase in number takes place, only from before back- 



1 It is advisable on account of priority to use the term Branchiostoma in place 

 of the more familiar term Amphio.\us. as Prof. K. A. .\ndre\vs has recently pointed 

 out in his paper on An Undcscribcd Acraniatc. .Studies Hiol. I.ab. Johns Hopkins 

 Univ., V, no. 4, 1893- 



Fig. 6.— Two thread- 

 cells. I, une.xplotled, 

 showing two crossing 

 layers of the threads. 2, 



