958 ; REPORT—1896, 
totally different in structure from the thyroid of all other vertebrates, though 
resembling the endostyl of the Tunicates. 
The thyroid of Ammoccetes may be described as a long tube, curled up at its 
posterior end, which contains in its wall, along the whole of its length, a peculiar 
glandular structure, confined to a small portion of its wall. 
A section through this tube is given in fig. 7, and shows how this glandular 
structure possesses no alveoli, no ducts, but consists 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 
MUCO-CARTILAGE OPERCULUM 
BRANCHIAL Zs 
Thyroid (Ammoccetes). Thyroid (Scorpion), 
end of the cell, farthest away from the lumen of the tube. Such a structure is 
different form that of any other vertebrate gland. Its secretion is not in any way 
evident. It certainly does not secrete mucus or take part in digestion, and for a 
long time I was unable to find any structure which resembled it in the least 
degree, apart, of course, from the endostyl of the Tunicates. 
Guided, however, by the considerations already put forward, and feeling 
therefore convinced that in Eurypterus there must have been a structure re- 
sembling the thyroid gland underneath the median projection of the operculum, 
I proceeded to investigate the nature of the terminal genital apparatus under- 
lying the operculum in the different members of the scorpion family, and reproduce 
here (fig. 8) the figures given by Blanchard ! of the appearance of the terminal male 
genital organs in Phrynus and Thelyphonus. Emboldened by the striking appear- 
ance of these figures, I proceeded to cut sections through the operculum ot the 
European scorpion, and found that that part of the genital duct which underlies 
the operculum, and that part only, contains within its walls a glandular structure 
which resembles the thyroid gland of Ammoccetesin a remarkable degree. A section 
is represented in fig. 7, and we see that under the operculum in the middle line is 
situated a tube, the walls of which in one part on each side are thickened by the 
formation of a gland with long cells of the same kind as those of the thyroid; the 
nucleus is spherical, and situated at the farther end of the cell, and the cells are 
arranged in wedges, so that the extremities of each group of cells come 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 
1 Blanchard, L’ Organisation du Régne Animal. 
