THE EVIDENCE OF THE THYROID GLAND 



I 9 I 



operculum in the old extinct sea-scorpions, Eurypterus, Pterygotus, 

 etc., belonged to the type of Thelyphonus, rather than to that of 

 Limulus or Scorpio. In Fig. 78 I give a picture from Schmidt of the 

 ventral aspect of Eurypterus, and by the side of it a picture of the 

 isolated operculum. Schmidt considers that there were five branchiae- 

 bearing segments constituting the mesosoma, the foremost of which 

 formed the operculum. Such operculum is often found isolated, and 

 is clearly composed of two lateral 

 appendages fused together in the 

 middle line, of such a nature as to 

 form a median elongated tongue, 

 which lies between and separates 

 the first three pairs of branchial 

 segments. This median tongue, 

 together with the anterior and 

 median portion of the operculum, 

 concealed, in all probability, accord- 

 ing to Schmidt, the terminal parts 

 of the genital organs, just as the 

 median part of the operculum in 

 Phrynus and Thelyphonus conceals 

 the complicated terminal portions 

 of the genital organs. The posterior 

 part of the operculum, like that of 

 Phrynus and Thelyphonus, carried 

 the first pair of branchiae, so Schmidt 

 thinks from the evidence of markings 

 on some specimens. 



Apparently an opercular ap- 

 pendage of this kind is in reality 

 the result of a fusion of the genital 

 operculum with the first branchial appendage in forms such as the 

 scorpion; for, in order that the tergal plates may correspond in 

 number with the sternal in Eurypterus, etc., it is necessary to 

 consider that the operculum is composed of two sternites joined 

 together. Similarly in Thelyphonus, Phrynus, etc., this numerical 

 correspondence is only observed if the operculum is looked upon 

 as double. 



A restoration of the mesosomatic region of Eurypterus, viewed 



Fig. 78. — Eurypterus. 



The segments and appendages on the 

 right are numbered in correspon- 

 dence with the cranial system of 

 lateral nerve-roots as found in verte- 

 brates. M., metastoma. The sur- 

 face ornamentation is represented 

 on the first segment posterior to the 

 branchial segments. The opercular 

 appendage is marked out by dots. 



