26 T>r Gashell, The Origin of Vertebrates. [Nov. 25, 



Phrynus or of Eurypterus with its anterior genital part and its 

 posterior branchial part. 



Further, the nerve which innervates the hyo-branchial portion 

 of this segment is the Vllth nerve, and Miss Alcock 1 has shown 

 that a branch of the Vllth passes along near the ciliated groove to 

 extend ventrally tailwards as far as the thyroid itself extends ; so 

 that looking upon the whole of this segment as the opercular 

 segment we see that the Vllth nerve was originally the opercular 

 nerve just as the IXth was the nerve of the first branchial appen- 

 dage and the Xth of the remaining branchial appendages. 



The comparison of the thyroid segment of Ammocoetes with 

 the anterior genital half of the operculum in Eurypterus and 

 Thelyphonus means, that the thyroid gland must resemble closely 

 the median part of the genital apparatus lying under the oper- 

 culum of these latter animals ; and indeed the resemblance both 

 in position and in structure is extraordinarily close. 



The thyroid of Ammocoetes consists of a long tube curled up at 

 its posterior end, that tube containing along the whole of its 

 length a very peculiar glandular structure which is confined to a 

 small portion of its wall. 



This glandular structure has no alveoli, no ducts, but consists 

 simply of a column of elongated cells arranged in a wedge-shaped 

 manner, the apex of the wedge being in the lumen of the tube ; 

 each cell contains a spherical nucleus situated at the very extreme 

 end of the cell furthest away from the lumen of the tube. Such a 

 structure is different to any other vertebrate gland, its secretion is 

 not in any way evident, it certainly does not secrete mucus or take 

 part in digestion, and it seemed impossible to compare it with any 

 other known glandular structure. 



Looking however at the pictures given by Blanchard of the 

 genital apparatus of Thelyphonus and Pbrynus it is striking to 

 see how closely in appearance the median part of the genital 

 mechanism resembles the thyroid gland of Ammocoetes, and upon 

 cutting sections of the Scorpion (the only material available) I 

 found under the operculum in the middle line a tube, the walls of 

 which in one part were thickened by the formation of a gland 

 with long cells of the same kind as those of the thyroid, the 

 nucleus being spherical and situated at the very further end of 

 the cell, and the cells being arranged in wedges so that the 

 extremities of each group of cells came to a point on the surface of 

 the inner lining of the tube. This point is marked by a small 

 round opening in the internal chitinous lining of the tube ; these 

 cells form a column along the whole length of the tube just as 

 in the thyroid gland, so that the chitinous lining along that column 

 is perforated by numbers of small round holes. 



1 Miss Alcock's paper "On the cranial nerves of Ammocoetes " is not yet pub- 

 lished but will appear shortly. 



