1895.] Dr Gaskell, The Origin of Vertebrates. 25 



not by means of capillaries as in the higher Vertebrate, but by 

 narrow channels caused by the subdivision of a large blood space, 

 just as in the branchiae of Limulus or Scorpio. 



The conclusion then is that the ancestor of Vertebrates breathed 

 by means of lamellar branchial appendages which had united 

 together so as to form a scorpion-like body surface, and that the 

 nerves to these appendages were the Vllth, IXth and Xth. 



In fact, the method of formation of the scorpion from Limulus 

 as given by Macleod precisely illustrates the manner in which 

 a respiratory chamber like that of Ammocoetes may be compared 

 in the first place with that of Eurypterus, and so with that of 

 Limulus. To this point I will revert later on. 



The history of the thyroid and of the Vllth nerve. 



If we compare the ventral surface of Eurypterus with that 

 of Ammocoetes, we see that the comparison of the branchial seg- 

 ments with each other does not complete the ventral surface in 

 either animal, for in each case a triangular projection passes down 

 in the middle line and separates the 1st three branchial segments 

 from each other. This projection in the case of Eurypterus is the 

 middle projection of the operculum, and in the case of Ammocoetes 

 is a middle plate of muco-cartilage underneath which lies the 

 thyroid gland. 



In all scorpion-like animals the respiratory segments of the 

 body are terminated anteriorly by the operculum, just as the 

 operculum is the first of the lamellar appendages of Limulus, and 

 in all cases the operculum carries the terminations of the gene- 

 rative organs, while in a great number of cases, as in Eurypterus, 

 Thelyphonus, Phrynus, the operculum is double, the posterior part 

 being gill bearing, while the anterior part carries the organs in 

 connection with the external orifice of the generative organs. 



In Ammocoetes we find that the first of the branchial segments 

 (the hyo-branchial) is not like the rest of the branchial segments, 

 the gill surface being confined to the posterior part of the segment 

 while the anterior part of the segment carries on its inner surface 

 the ciliated groove called by Dohrn the pseudo-branchial groove ; 

 this groove as is well known passes from the dorsal to the ventral 

 side and there joins with its fellow of the other side to form the 

 so-called duct of the thyroid gland ; this bilateral gland lying 

 under the ventral median tongue of muco-cartilage which separates 

 in so remarkable a manner the cartilaginous basket-work of either 

 side. In position, then, the segment composed posteriorly of the 

 hyo-branchial, anteriorly of the ciliated groove and the median 

 thyroid corresponds exactly to the operculum of Thelyphonus or of 



